Okay, but, while I agree 100% with everything OP has said, they're missing one glaring, screaming clarification that all of the posts about this I've seen keep missing, and I need to say it.
There is no actual "CP" in AO3.
First of all, do not call it CP. It is not called CP. Every advocacy and regulatory body and TOS on the fucking internet does not call it CP.
CP stands for Child Pornography.
This was the lingo of the 90's. This was the term chosen in time because it produces a very visceral, upsetting reaction in people. It's a term meant to induce disgust.
It is also a term that carries very, very, very unfortunate implications.
It is 2022, and we have all as society agreed that Child Pornography doesn't exist, per se, because "pornography" describes an industry that adults participate in. Ideally, willingly, but there's abuses and excesses, like in all industries, where people are coerced. Pornography is like retail: some people do it because they enjoy it, some people do it because they have to pay bills, some people do it because someone is coercing them into it.
But the keyword is "participate".
Pornography is made by adults, for adults, and as such, all adults have a level of agency in it.
CSAM, or Child Sexual Abuse Material, is the proper term to refer to what people think of, when you say "CP". We changed the terminology, because children have no agency. It is not pornography. It is material explicitly recording and displaying abuse. Children cannot consent to sexual activity.
Children cannot participate, the way adults do, in pornography.
The other thing, to keep in mind, is that AO3 does not host CSAM. At all. It is against their TOS already. No website with servers in the US, or in a country with any kind of economic or diplomatic relations with the US, does. Because it's against the law. See the fiasco with the AO3 volunteers that were sent actual CSAM materials and the way AO3 handled that.
AO3 does not host any kind of CSAM.
When you get that context, you realize that any cries to remove material from the archive are not being done in good faith. And therefore any and all replies to those, are pointless.
The hot button material that people are upset about are fictional works depicting fictional minors engaged in any level of sexual activity, thus tagged "Underage"; and specifically, a subjection of those works, that are also fictional works depicting fictionalized versions of real people, thus tagged "RPF".
Whether you think this material should or shouldn't be allowed in the Archive, is one thing. (I personally don't particularly find it to my taste and would not be super heartbroken if AO3 decided to remove it, but that doesn't matter, because it is allowed and fits within the guidelines and falls under the mission statement of AO3, and has since inception, so every single person who is currently using AO3 has, at least in theory, accepted that. If people tell you "they didn't know" this or try to frame it like it's something new? Those people are either lying to your face or idiots who didn't read the TOS before joining the site.)
But you cannot, in good conscience, call this "CSAM" or related terms. You cannot. It is literally and explicitly not.
The reason CSAM is illegal and bad and terrible and consistently accepted as one of the worst possible things to exist in the world, is because its existence depends on the abuse of a child. Its very existence is confirmation that a crime has been committed.
The point is not that CSAM is "disgusting". Even though it is, 100% and if you don't think it is, you're a fucking idiot. The problem with CSAM is the inherent harm its creation requires. That's why it's illegal basically everywhere. That's why every TOS is written to exclude its presence explicitly.
The content currently in AO3, "Underage RPF", does not match that definition. Does not cause the harm that CSAM does. The morality of Underage RPF can be up for debate. The actual/potential harm of Underage RPF can be up for debate.
But you can't have any of those conversations if you don't start from a clear understanding that it is not "CP", that "CP" is not even a "real" term anymore and should instead be considered a red flag, used primarily to antagonize and demonize the opposition.
OP is absolutely right in tracking how accusations of this nature have been used historically to target and censor and destroy queer communities. But OP is also playing into the game by not rejecting the mislabeling of content, because THAT too is what they want. When they accuse AO3 of hosting CP, and you skip clarifying that, no, actually, AO3 does not host ANY CP at all in the first place, you've already lost. You've already handed them the win, because any and all arguments become void. You're defending CP in their eyes, and they will frame it as such. And if you're defending CP, then anything you say is irrelevant. Or worse, if you're defending CP, that means everything else you're defending (queer people, freedom of expression, writers, FICTION, etc.) is also as suspect and as inherently bad as the CP.
Otherwise you wouldn't defend it in tandem.