EDIT: Unfortunately, some here continue to deny this and it's widely known that Mastodon (and the top servers) have been infested with CP and it is still big in Japan [0][1] for years.
Excluding the default instance, the top 3 Mastodon servers (Pawoo, baragg (d_o_t) net, and mstdn (d-o-t) jp) are the biggest and have tons of CSAM.
The common denial, Exhibit A. [2]
[0] https://ethanzuckerman.com/2017/08/18/mastodon-is-big-in-jap...
[1] https://htxt.co.za/2023/07/child-abuse-material-is-thriving-...
A lot of bots are posting pornographic content or selling illegal items on that platform. And since many of the bots are "verified", it is harder to filter them out completely. The whole thing is a mess at this point.