>but nearly all of it is actively filtered via the IWF lists.
You wish. Those lists can only filter specific sites. Not the myriad of child porn that is shared privately. So those list based blocks might in fact protect some users who inadvertently clicked on the wrong link, not knowing where it would lead. But people actually seeking out this content do not care.
It still would be effective to just inform providers and domain registrars[0].
I'm involved in running a small, non-commercial website that offers instant chat rooms without the need to sign up first. At some point the kiddly diddlers found out about it, and used it as basically a bootstrap tool for their activities. We had to play cat and mouse with them for a while, but have improved our system where most such things get automatically detected now or at least marked as suspicious, and the rest is handled by humans. So I am in the unfortunate position to know a thing or two about how these people operate, or at least those who abused our service.
After they initially find each other, either to "trade" one on one or have a group thing going, they use services such a mega.nz[1] and other file hosters with ever new download URLs, or switch over to messenger services such as kik[2], tox[3] and discord[4] in particular, and use email (in particular protonmail these days) as a backup to finding each other again should their other accounts be taken down.
TOR also plays a minor role, of course, but TOR's importance is usually overstated in public discussions about child porn, at least as far as I have seen, and we've rarely encountered people sharing hidden services; it's mostly about using TOR as an anonymizing, free "VPN"[5].
The amount of content (aka child abuse media) they share is tremendous. Them sharing 10s of GBs with a few mega links in one go was common.
[0] We had this discussion in Germany ten years back, when the government proposed such filter lists that the ISPs should then enforce. There was a big opposition to that, as people thought the content should be actually removed and perpetrators prosecuted instead of just "hiding" it, and also establishing such a system would make all kinds of other groups eager to get courts to block other content with it, such as alleged piracy sites. Oh, and of course the lack of accountability and transparency was a big issue too.
Turns out most providers will cooperate in taking down content, and the rest can be "made" to cooperate. The German group AK Zensur (NGO) ran an experiment back in 2009, and contacted providers based on leaked government child porn filter lists (mostly from Scandinavian countries). Most providers responded within 12 hours, removing illegal content, but more often than not they did not actually find child porn whatsoever (which the AK Zensur then verified). Most providers further said they had not been contacted previously (good job, law enforcement).
https://ak-zensur.de/2009/05/loeschen-funktioniert.html (German)
[1] mega.nz's model of plausible deniability means they also cannot identify content because they never see the content unencrypted (not even a hash of the unencrypted content). So they can only take down individual URLS and accounts, but cannot take down exact copies of the same file uploaded to other accounts. This, combined with the ease of making new accounts, each of which gets 50GB storage at first, and the feature to "unshare" (invalidate share links; allows people to share content for sometimes just a few seconds, so other people can import it into their own accounts and download later), makes mega very popular not just with content pirates, but also with people sharing child porn.
[2] Easy to use, and easy to use group chats.
[3] Harder to set up, but it claims to allow quasi anonymous decentralized communication incl file sharing if you put it behind a VPN or TOR. We've encountered "how to setup tox" inforgraphics in the past.
[4] Discord used to be a bit wild west, being a new service which hadn't figured out a lot of things yet (kinda like us at the beginning), and probably underestimated the problem (also kinda like us in the beginning). They also made it pretty hard to actually report abuse (something like, "actually join the server, switch to developer mode, toggle some obscure other preference, copy the internal channel ID and write us an email containing that information"). Tho it seems they are doing a lot, lot better recently in banning "servers" involved in child porn.
[5] Which put us in the unfortunate position to proactively ban TOR users from chatting (they can still read). 99% of TOR traffic was either outright malicious stuff like child porn, or spam. Sorry to the remaining less than 1% who played nice.