Nothing wrong with the matrix spec. I've ran a matrix homeserver for several years now and have never had issues with illegal material. It's free, open source, and self-hostable. If you have an issue with matrix.org that's a different issue. Don't use matrix.org servers. In fact, they have made it a point in their literature that they want you to use your own servers, they preach against centralization, and point out that matrix.org is an entryway to get you started, not an endpoint.
Yes I watched your video. So many wrong comments from ignorance. And blaming a protocol for the behavior of people? Another commenter here pointed out how Tor could be blamed in the same way. Why not just do away with the internet then? Ridiculous.
Absolutely not. I left those communities because of what was in them, I will not be seeking them out on purpose just to show a bunch of CSAM to people and probably incriminate myself.