Too many people are ignorant of this.
I have a belief (not based in law, just my personal feeling) that once you post something in a conversation in a public forum, you no longer have any natural right to control it. By posting it in a conversation on the public internet, you have, in a practical sense, waved any right to control it. That is a part of a conversation that belongs to the public, and you gave your comment away freely. It is public record. You cannot demand that it be forgotten or erased any more than I can demand that something I said to my friends yesterday be forgotten and erased.
If I hosted a forum, I would make it clear that this is the policy, and I would not allow people to delete comments that they posted. Edits would be allowed, but the history would be available. Deletions would only ever happen if I was legally compelled.
This all gets complicated if someone posts private information about a third party. I would rapidly delete such posts and ban such users. The third party never consented to anything, so it’s not the same.
Unless I’m mistaken, this is just a matrix server. If you were using Dendrite, you wouldn’t say “hey let’s try dendrite,” you would say “let’s try Matrix, here’s my server.”
At least, that’s how I would say it. Server software choice is details they would not care about and would likely cause confuwusion.