Arguably, this isn't your problem. In most circumstances, you don't give loot to specific characters - you give it to the party, and let them sort out how it gets distributed.
Yes, this could theoretically lead to one player hogging all the magic items, but that's a solved problem; As long as your players are aware that they can negotiate how stuff gets shared around, they'll generally spot unfair situations, renegotiate as necessary, and generally agree on equitable solutions on their own.
If one player's lack of magic items is dragging the party down, that's the whole party's problem, and most players will realise that pretty quickly; Even the most selfish of players will agree it's a good idea to share the wealth if it improves their own survivability.
If magic items have already made one player much more powerful than the others - Well, it's not like magic items are bound to the first person who picks them up; Your players can always just redistribute them where they'll be more useful.
If your players don't find an uneven distribution of loot to be inequitable - Well, then there's no problem to solve; Everyone's already having fun, and that's what the game is all about.
So, to repeat myself, this isn't your problem: Working out how to distribute loot is just another part of the game. Heck, some players find it one of the most fun parts of the game: Working out how to effectively use available resources is core gameplay, and deciding how magic items should be distributed for best effect is part of that.
Version 1.1 Player's Guides
The “+1 rule” is shorthand for the restriction on being able to use the Player's Handbook plus one extra source to build your characters for the Forgotten Realms campaign.
WoTC has two different campaigns that utilize different limitations (currently v1.1).
- Forgotten Realms maintains the "+1" rule (see below)
- Eberron does not and includes additional resources and limitations.
Forgotten Realms (Waterdeep/Dragon Heist)
From page 2 of the 9.2 version of the Adventurer's League Player Guide (ALPG).
Forgotten Realms characters can choose race and class options from the Player’s Handbook and one other resource—a rule called “PHB+1.” Additional resources include the following products:
- Elemental Evil Player's Companion (EEPC)
- Sword Coast Adventurer's Guide (SCAG)
- Volo's Guide to Monsters (VGM)
- Xanathar's Guide to everything (XGE) [includes Tortle package]
- Mordenkainen's Tome of Foes (ToF) [Chapters 1-5 only]
- Locathah Rising (LR)
Additionally, when selecting an additional source, the following variant or optional rules are available when you create your character:
- Variant Human Traits (PHB)
- Half-elf and Tiefling Variants (SCAG/ToF)
- Option: Human Languages (SCAG) [This option isn't subject to PHB+1]
- Blessing of Corellon (ToF) [This option isn't subject to PHB+1]
NOTE: Races with flight at 1st level, and options from any resource other than those listed above aren’t available without specific campaign documentation (i.e., certs, etc.).
Eberron (There is no +1 rule per the ALPG linked above.)
From the 1.3 version of the Eberron Campaign Player's Guide (December 2019), which accompanied the Oracle of War campaign:
Oracle of War characters can choose race and class options from [see below] only (unless you possess campaign documentation providing additional options).
You may select the Variant Human Traits option from the Player’s Handbook.
- Player's Handbook (PHB)
- Xanathar’s Guide to Everything (XGE)
- Eberron: Rising from the Last War (ERftLW)
The previous guidelines used for the Embers of the Last War campaign, released before the publication of ERftLW, substituted the below sources for that book and did not allow any exceptions for use of other material:
- Wayfinder's Guide to Eberron
- Volo’s Guide to Monsters (Bugbears, goblins, and hobgoblins only) (VGM)
Best Answer
Ultimate Authority
Ultimately, the decision on appropriateness when writing CCC adventures is determined by the DDAL administration. Each Convention Created Content module must be submitted to them for approval before it can be played. Groups and individuals with a long track record of producing quality content are given more leeway.
Established Patterns
It works out very simply...
A variety of common magic items are a fairly recent occurence. To my knowledge, they have not been explicitly included in anything except DDIA-XGE: Underworld Speculation, the introductory module for Xanathar's Guide to Everything where they first appeared.
How many? One
As for how many magic items: one, always one - regardless of length. Hour-long introduction modules have none, and Epics sometimes have more, but neither are part of the CCC program.
I can speak from personal experience with the CCC program. I am indirectly affiliated with a CCC writing group; they submitted a module for a convention I staff for. It had two magic item rewards, both very thematic. They were told to pick one or the other by the DDAL administration.
A Note on Precision of Language
The question says...
...but it's not an accurate statement.
CCC adventures are sponsored by and affiliated with a convention they premiere at, but the creation process starts long before the convention takes place. They're not created freely by the conventions to "become certified" later. They are authorized by the DDAL administration to premier at a particular convention before writing even truly begins.