Yeah, fair. It can’t delete your messages to the extent a centralized system, and that’s an indication of the lack of centralized control? It’s a different threat model I think many find satisfying (though perhaps not most).

• Always read cited sources

in other words, devices don’t delete megolm keys after they’ve been used to decrypt history (which is why you can back them up and share them with your other devices in order to ensure that all your devices can read your history).

• Still I could only find:

Your username is stored indefinitely to avoid account recycling.

• Same for telegram. Most other messagers store only joins.
• They don’t control other homeservers. You never know if there is some homeserver/instance that stores everything in Matrix/Mastodon/Lemmy/PeerTube/Pleroma/whatever-else. Still I could only find:

Your username is stored indefinitely to avoid account recycling.

EDIT:

We will forget your copy of your data upon your request. We will also forward your request to be forgotten onto federated homeservers. However - these homeservers are outside our span of control, so we cannot guarantee they will forget your data.