This has been due to a very persistent spammer attacking the public network. Since adding policy servers (https://matrix.org/blog/2025/04/introducing-policy-servers/) as a way to preemptively block content and other measures (https://matrix.org/blog/2025/02/building-a-safer-matrix/) things have improved at least in public rooms - assuming the admins enable policy servers. We sincerely apologise to everyone who has got exposed to this, and are continuing to improve the trust & safety apparatus in Matrix (although could really do with funding for the Foundation to help folks work on this as their day job).