Because they can revoke they keys at any given time by any reason later on anyway.
That's fine. When you generate the keys, you understand that they might be revoked at some point.
"FOSS" is a corporate psyop to harm computer freedom and it shows it's effective given it convinced you it's the same as free software
Richard Stallman doesn't oppose the term, so I'm okay with saying FOSS or FLOSS. If a program is free/libre, it is also considered FOSS. There are differences, but they relate to the goals stated by those who prefer each term.
Tekegram effortlessly convinced you they're not bad actors.
They didn't convince me of anything. When I choose to use it, I am aware of the risks involved. And I don't recommend that people use it if they want privacy. I mentioned before that I moved my conversations to SimpleX.
Whoever maintains a proprietary fork doesn't matter as the client itself is still being proprietary.
The client is definitely free, not proprietary. It grants you the 4 essential freedoms. Yes, if the API keys are revoked, the program becomes useless, but it doesn't change its status as free software. For example, the NewPipe Android app has broken many times in the past because Google changed how videos are retrieved. But when it broke, it didn't suddenly become nonfree, it just became useless.
And every single one of them is just as proprietary.
They are free. A free program can talk to nonfree network services. For example, many people use git, a free program, to download or upload files to nonfree services like GitHub.
It is a problem as Telegram implements a form of DRM into it, which goes against freedom 0
You're trying to invent an interpretation of the GPL that doesn't make much sense. Essentially, what you're saying is that any program accessing any network resource via API is nonfree unless said network resource is also free. Since the client and server are different entities, I don't think this assumption is correct.
which is XMPP
XMPP is just a worse version of Matrix. You're at the mercy of whoever hosts your server. I don't want to depend on someone else. I've already experienced being banned from a Matrix server because the admin didn't like something about me. I want a platform that doesn't even allow the possibility of removing a user's account or tampering with their messages. Currently, I use SimpleX. I also tried Session, but didn't like that it doesn't support large group chats.