The cards are probably not edited with errata changes.
The original 2014 printing was redone in 2015, mostly to address rounding the corners on sharp cornered v1 cards and also to add an indication of whether a spell was concentration or not (that had apparently been omitted from the info on the cards in v1). Even by then, errata were being compiled and corrected for the core books, but I find no official word on those updates ever being considered by GF9 when updating the cards. If it were me in charge of it, that certainly would have happened.
If you want to use spellbook cards and you must be absolutely certain the cards incorporate all the necessary and most up to date text, you may need to create your own, or use some resources online to help you do so.
What follows is information gathered online, decide for yourself how reliable any of it really is.
GF9 has an official online store page for those cards. Most of the currently available sets are designated "2018 edition" in the online store, though there's no statement about how often they create new editions, exactly what is updated in each edition, or how to distinguish one edition from another (note on the bottom next to the UPC code, also cover art is different) when ordering from other sources.
The Arcane cards claim to now include spells from the SCAG, so the deck was updated since the first PHB printing, but not necessarily in ways indicated by the PHB errata.
There's no mention of errata or which PHB printing the card text is based on, so Gale Force 9 doesn't make a clear assertion that the 2018 edition incorporates errata. They are adding spells to some decks as well as adding blank DIY cards, and that seems to be what constitutes the update.
The cards for Xanathar’s Guide to Everything contain spells previously released in the Elemental Evil deck. There are some differences in some spells that could be considered errata corrections.
Several Amazon reviews for the Arcane deck assert the errata document wasn't considered in the 2018 update to the cards, but the reliability of the reviews are open to question. No photos of known errors either present or corrected in the cards are offered as proof.
Ted at Nerd Immersion on Youtube has a review/unboxing of the "version 3" cards released in 2018. There are a lot of updates, but Ted's review doesn't ask or answer the question of whether errata were considered in the version updates.
In this unboxing video the Acid Splash card is clearly shown. It does not have the change in wording from the 2018 2.0 version of the errata. It looks like a version 3 deck, because the box art image is in portrait orientation, v1 and v2 had landscape orientation covers. It's possible the version 3 deck includes corrections from the 1.22 version of the errata from 2017 (which would not have a change for Acid Splash ) but I am not very hopeful.
“No hidden rules” is just what it sounds like, and what several other answers say: the rules are what’s printed on the page, and you have access to them.
That wasn’t always the case.
You mention Mao as “not an answer”. Oddly, that’s a good model for how many games used to work: the GM knows the rules, and the players don’t. or at least they only know some of them.
Up through about 2000, there was a weird taboo around players reading books that were intended for the GM. For example, from the preface to the AD&D DMG:
What follows herein is strictly for the eyes of you, the campaign referee. As the creator and ultimate authority in your respective game, this work is written as one Dungeon Master equal to another.
This sentiment shows up pretty often. So, everything in that book is intended to be secret from players. How that played out at any given table varied wildly, just like you’d expect, but the default version of D&D was a lot like Mao. One person had a rules reference, everyone else learned by doing.
Some things, that’s clear cut. For “how does alcohol work?” the answer is obvious: ask your DM. Because it’s in the DMG. But, more relevant to your question, sometimes it was more insidious. For easy example, take charm person.
I’m not going to copy the AD&D PHB text here, because there’s a lot of it. It’s basically what you’d expect: charm person for magic users refers back to the Druid’s charm person or animal, plus a list of things that are considered “persons”. Charm person or animal works about like modern charms: Subject regards you favorably, new saves if you threaten it, breaks if you attack it. It also had a periodic save with frequency based on intelligence.
Then, the AD&D DMG has this to say:
Charm Person: Attacks causing damage upon the subiect person will cause a saving throw bonus of +1 per hit point of damage sustained in the round that the charm is cast.
If you’re playing “correctly”, you never see this rule. You might notice that charm never works in combat, but the mechanical basis is secret.
The DMG has a half-dozen similar entries for each spell level, for each class. So, if you’re coming from that background, “there are no secret rules” is extremely meaningful. It’s also revolutionary (or would be if that hadn’t been the new normal since the mid-noughts).
Most games before D&D’s third edition that I have read had similar attitudes. I’m not claiming that any change was WotC’s doing, but that was the timeframe and 3e reshaped the landscape in a big way.
So, bottom line: yes. There were secret rules.
——-
Now, all that said, there was never any D&D police force that would haul you off to RPG jail if you read the DMG as a player. You can definitely find accounts online (horror stories usually) of DMs becoming very upset if they learned a mere player had dared read DM material, but I suspect most groups didn’t really care.
Best Answer
There are a few changes - some minor, some more significant - in the latest printing of the PHB that are not noted in the 2018 errata PDF:
Action Surge
The fighter's Action Surge feature previously read (PHB, p. 72; emphasis mine):
The bolded phrase was removed in the latest printing as of November 2018, and the Action Surge feature description now reads:
This change is confirmed in the January 2019 edition of the Sage Advice Compendium:
Rules designer Jeremy Crawford also confirmed the change in the November 27, 2018 edition of Dragon+ (in which he discussed the errata), describing it as "unhelpful helper text" that caused people to interpret it as saying the opposite of what it actually meant. (Thanks to CTWind's answer here for the timestamped link!)
Awakened Mind
In previous printings of the PHB, the Great Old One warlock's Awakened Mind feature read (PHB, p. 110; emphasis mine):
However, the description of the Awakened Mind feature in the latest printing of the PHB reads (emphasis mine):
Of note, this change makes it so that Awakened Mind's telepathy is more clearly one-way rather than two-way. This matches the rules intent as indicated in one of the earliest versions of the Sage Advice Compendium in 2015 (v1.01), and still listed there in the 2019 Sage Advice Compendium (v2.3):
The fact that the wording of the feature was changed from previous printings of the PHB, however, is not mentioned in the errata PDF or in the Sage Advice Compendium entry.
Climbing, Swimming and Crawling rules
The rules on Climbing, Swimming, and Crawling (PHB, p. 162) originally said:
As this question by Daniel Zastoupil and its answers note, there was technically some ambiguity about whether this meant having a climb/swim speed was sufficient to ignore the extra movement costs, or whether the creature needed to use those speeds in order to not have to spend extra movement.
However, as of November 2018, the rules on Climbing, Swimming, and Crawling now state (emphasis mine):
This change to the PHB (and basic rules) is not mentioned in the errata.
Clone
The first sentence of the clone spell description (PHB, p. 222) originally read (emphasis mine):
However, as of November 2018, the clone description now begins (emphasis mine):
The fact that it simply says "Medium" instead of "Medium or smaller" means that it's now impossible to use the spell to clone any Small creature. This seems like an oversight (presumably the intent was to prevent cloning Large or bigger monsters with it), in which case a future errata may fix it accordingly.
This added size specification is not mentioned in the errata PDF.
Special mentions
There are 2 "non-changes" in the 2018 PHB errata that actually misleadingly seemed like a change at first:
Deflect Missiles
The last sentence of the monk's Deflect Missiles feature (PHB, p. 78) originally read:
The first PHB errata in 2015 summarized the change as follows (and the same summary remained unchanged through the 2017 errata):
Instead of summarizing the change, the 2018 errata states the exact phrasing to be used - which oddly leaves out the information about the range of this attack that was added in previous errata:
This leaves out the information about range added in previous errata.
However, the last sentence of the monk's Deflect Missiles feature actually currently reads (emphasis mine):
This wording is seemingly unchanged from when the range of the attack was initially added in the 2015 errata. It appears that the 2018 errata simply incorrectly quotes the end of the final sentence of the feature, matching the version in the first printing of the PHB rather than the latest printing.
Martial Adept
The Martial Adept feat (PHB, p. 168) originally read, in the first printing of the PHB:
(Thanks to Mindwin's question here for the quote.)
Naturally, people interpreted this as meaning that the feat was better for Battle Master fighters (PHB, p. 73-74) than for others, since Battle Masters gained an extra superiority die of the same size as their existing superiority dice (all of which scale with their level).
The very first errata to the PHB (in 2015) summarized its change to the feat as follows (and this summary remained unchanged through the 2017 errata):
Some people took this to mean that the superiority die from Martial Adept still scaled with a Battle Master's regular superiority dice, with the only perceived change being that the superiority die scaled with Battle Master levels whether they were taken before or after taking the feat.
However, the wording of the feat in all printings since that 2015 errata is actually identical to the current (2018) version of the Martial Adept feat (this Reddit thread shows the same wording as below); the 2018 errata simply includes an exact quote of the change now.
The relevant portion of the Martial Adept feat now says (emphasis mine):
What this actually means is that Martial Adept's additional superiority die doesn't scale with Battle Master levels; the character with the feat simply has a separate d6 superiority die. (Battle Masters with the feat can expend either their regular superiority dice or their extra superiority die from the feat to use their maneuvers.)
It's arguable whether people were just misinterpreting how the errata previously paraphrased the change or whether the 2015-2017 errata misleadingly paraphrased it. (Mike Mearls' incorrect tweet supporting the misconception certainly didn't help.) Either way, it's now clear because the errata PDF quotes the exact wording of the change instead.