The premise of your question is somewhat incorrect. Level 20 is a standard progression limit only in 3e and 5e.
In First Edition AD&D, there is no level limit. Specific class advancement tables describe advancement from anywhere from 29 levels (cleric) to 9 levels (fighter) but only for purposes of showing how high certain abilities can go, they all note you can go on up infinitely from there. The big differentiation is "name level," which is usually in the 9-10 range, where the character stops getting as large level advances in hp and starts focusing on building kingdoms and whatnot.
In BECMI D&D (Red Box Basic), you can go up to level 36, and there are level breaks from Basic (up to level 3), Expert (up to level 14), Companion (up to level 25), Masters (up to level 36), and Immortal (past that, cashing in XP for power) with differences in those levels of play.
In Second Edition AD&D, advancement is described on convenient charts up to level 20 but there is no limit, with a breakpoint at level 9-10 where you stop getting full hit points with each level. It has a section in the DMG about how play gets harder to be satisfying at higher levels and that you probably need to shift campaign styles. In terms of playstyle recommendation past 20 it says
Consummate skill and creativity are required to construct adventures for extremely powerful characters (at least adventures that consist of more than just throwing bigger and bigger monsters at the nearly unbeatable party). Very high level player characters have so few limitations that every threat must be directed against the same weaknesses. And there are only so many times a DM can kidnap friends and family, steal spell books, or exile powerful lords before it becomes old hat.
It then recommends retirement as an endgame.
In Third Edition D&D, advancement is described up through level 20, with levels past that described in "an upcoming rulebook." It was lightly treated in the DMG but then more fully in the Epic Level Handbook in the 3.5e days. 3.5e and Pathfinder are lightly changed derivatives of 3e by design and so aren't really different editions with different ideas driving them as far as this goes. The Epic Level Handbook describes its intention, which is to change playstyle from the level 1-20 model to being legendary, allowing PCs to "wield powers that other characters (even 20th-level ones) can only dream about." It notes that PCs may have had the thrills of running nations and political machinations come and go and this is their gateway to discovering the secrets of the universe, plugging into the primal cosmic battles, etc.
You may also want to review the 3.5e DMG's discussion on epic characters and why attack and save bonuses cap out at 20 on p.207. (Summary: too many attacks causes slog and too much disparity between faster and slower save and BAB progressions causes balance issues). Also on p.210 they explain that many classes have been balanced assuming that 1-20 progression and that balancing classes for infinite progression is way harder.
In Fourth Edition D&D, the level limit is 30. There is no implied "soft cap" at 20. Play is grouped into rough "tiers" from the heroic (1-10) to paragon (11-20) to epic (21-30), but it is a continuum.
In Fifth Edition D&D, the game describes four tiers of play (1-4, 5-10, 11-16, 17-20) with "epic boons" available after level 20.
From this, you can take away several things.
The nature of play changes with level. Kicking down the door and killing something works well as a low level adventure and less well as a high level adventure, due to both repetition and the powers and abilities available to higher level PCs and foes, so shifting to more political or grander-scale adventures becomes desirable. Some editions formalize this with tiers, others say "low level, high level, and very high level", etc. What level that is varies by the specific D&D edition and its core rules.
There are a variety of "soft caps" and "hard caps" across the editions - tier boundaries, name level, etc. Only in 3e and 5e is 20 specifically a meaningful number that one might describe as a levelling cap (with later progression options). These numbers are not based on some arcane math but on when the designers feel like gameplay breaks down under its prior level paradigm. 4e is reusing the "epic" term but there is a continuum from 1-30 where epic can't be considered a meaningful cap, even a soft one, it's a breakpoint like the one at level 10. It's basically just using previous edition words for that level band to comfort people.
"Epic Level" play is a 3e thing based on a very specific product and terminology coined for 3e. Most references you've seen to "post-20 play" and "epic" are just an outgrowth of 3e play specifically. You are seeing something "across all of D&D" which isn't really across all of D&D.
Since versions of D&D mostly share certain rule similarities, the breakpoints of power - mostly cemented by what spells become available (fly, teleport, wish, etc. change the dynamics of the game by their availability) tend to be in around the same spots. So short of devising new things (tenth level spells, epic powers, etc.) versions of D&D that use the traditional spell advancement of "a new spell level every couple character levels" cap out spell power right before 20, where then it becomes a game of "more, but not really different" without additional rules that are pointless to include in core books where 99% of people don't ever get up to level 20 anyway. But this means that the around-level-20 breakpoint isn't really deliberately designed, it's more of an inevitable endgame of the spell system, unless you deviate from it (as 4e did). Even in BECMI, the Master rules (level 26-36) are only 32 pages long and are basically some new spells and then siege engine rules. When I played Basic no one ever went past Companion because the game got pretty weird and uninspired there.
Given a class-and-level system of D&D's kind, and the kind of Vancian casting powers traditionally available at levels around 5 (fly, fireball), 10 (teleport, raise dead), 18 (wish, miracle) then you get a similar need to change playstyles at those milestones, with 1-5 being your gritty stuff, 6-10 being (super)heroic, above that needing to change more to political and larger scale concerns to keep challenge and interest, and around 20 becoming a point of diminishing returns where you need to do something different to maintain challenge and interest given how spells etc. cap out there. BECMI Master pushed this past to 36 and got boring for that whole range; 4e went to 30 by discarding the Vancian tradition other editions share.
So it is incorrect to say that 20 is a soft cap across most editions, but this is the reason behind it in 3e/5e and the other "caps" and "breakpoints" and "tiers" in other editions in general. It's an emergent condition of the kind of ruleset D&D is and its historical trappings (Vancian magic being the most important) driving a change in playstyles at certain power inflection points. The designers explicitly talk about this in each edition's books regarding high/various level play.
If it needs to be stated more simply, 20 is not a magic number, it's just when having 9th level spells gets old.
Known History of the DCFS
Using Embrace- and Shun the Dark Chaos in succession, which combination allows a character to replace any feat they currently have with any feat they currently qualify for, appears to have struck some optimizers fairly quickly after the Fiendish Codex I was published. The first poster I found was HotSake, but he was actually beaten to the punch, and with more complete understanding of the utility of the combo by Zemyla. Zemyla specified that '...you can replace any feats you have...', which described that feats of all sorts are fair game.
As for using the combo for replacing a feat that you can somehow regain for a nominal cost, such as time, I first found skydragonknight's combo, which was also preceded by Sinfire Titan's combo.
(4/16/2017) The first usage of the term dark Chaos Feat Shuffle the I've found is by Wizard Random, with archerpwr using 'FC1 feat shuffle' about the same time.
So Zemyla announced the first reading of how to replace unnecessary feats with the DCFS, and Sinfire Titan appears to have boasted of the first repetitious replacing of a feat or feats via DCFS.
The Posts, in the Order Discovered
I don't fully recall if I read this first thread when the WotC 339 board still lived, as I was very new at looking for mechanical advantage at the time (I vaguely recall it, but none of the later ones personally), but this initially appeared to be the earliest reference to combining the two spells for their unintended consequences.
Better than retraining feats, at the cost of your soul.
HotSake |07-21-06, 12:01 AM| "Looking through the Fiendish Codex, I ran across the Embrace/Shun The Dark Chaos spells. This pair struck me as useful..."
However, a week earlier, this thread said the same thing:
Zemyla |07-13-06, 05:28 PM| "With the Codex of the Abyss book, you can replace any feats you have with other feats..."
Zemyla specified '...any feats you have...', and Chaos116882 explained to Tleilaxu_Ghola why it trumps Retraining.
This was originally the earliest I found explicitly for 'Infinite Feats' using DCFS:
Another Way to Get Infinite Feats?
skydragonknight |02-15-08, 10:49 AM| "This is another abuse of the Embrace/Shun the Dark Chaos Trick..."
But in this thread, 4 days previously, Sinfire Titan boasted, then delivered:
Book of Nine Swords - My God! What have they done???
Sinfire Titan |02-11-08, 02:11 AM| "...Heroics, Embrace the Dark Chaos/Shun the Dark Chaos. And they can even get Infinite feats with that last one and the Elder Evils book, if they know what they are doing..." |02-12-08, 11:54 AM| "...I take full credit for this loop:
- Devote the character to an Elder Evil, thus gaining 5 free bonus Vile feats.
- Embrace the Dark Chaos to swap them for Abyssal Heirator feats.
- Change Alignments and cease devotion to the Elder Evils (Game RUles check to see what Vile feats are to be lost, feats are no longer there, nothing happens, PC retains feats.)
- Shun the Dark Chaos to gain a new feat. If you are no longer Evil, your little ruling of them getting back the Vile feats won't work, as they no longer qualify (no longer Evil aligned, a requirement for every Vile feat).
- Shift alignment again, and repeat from Step 1.
Bam, infinite feats legal by RAW and without Pun-Pun loop..."
New findings (4/16/2017)
archerpwr mentioned 'FC1 feat shuffle' in passing:
n00b to epic. Help me make a gish?
archerpwr | 06-15-07, 06:45 PM | ...3) location bonus feats + FC1 feat shuffle for the following feats...
and Wizard Random mentioned 'the Dark Chaos Feat Shuffle' by name:
4 round Timeless Body, is it worth it?
Wizard Random | 06-17-07, 07:45 PM | ...Now while metapower is a bit restrictive there is always Reformation or the Dark Chaos feat shuffle.
Unfortunately, it looks like this was a known title, because nobody asked what it was or commented that it was a good name. Its usage is almost a year after Zemyla's noting of the utility of the combo, but I've found no earlier uses of the phrase.
The earliest possible encouragement to have fun appears in the foreword to the Original D&D Men & Magic (1974):
As mxyzplk answered, Swords & Spells (1976) includes a similar exhortation in its foreword, referring specifically to the DM's right to change rules to improve the game:
The adventure module In Search of the Unknown, included in the 1977 basic set, makes the explicit reference that the Dungeon Master is responsible for the players' enjoyment:
It also includes this specific advice in the tips for players:
The 1977 Holmes Basic rules themselves make one of the first references to tbe exact word "fun", again in the context of the DM changing rules:
Later, in the AD&D 1st edition Dungeon Master's Guide (1e), Gygax formally advises in the section "Approaches to Playing Advanced Dungeons & Dragons" that the goal of the game is to have fun, and he uses the word fun:
In this context, he's defending D&D's perceived lack of realism, its weaknesses as a simulation, by defending, as Gygax wisely often did, that D&D was a game intended for fun rather than to simulate the most precise outcome. By 1979, other RPGs had been created which attempted to be better than D&D by being more detailed or more realistic, and the result was often an incredibly tedious RPG that was no fun to play.