We've had per-room/per-message retention policies in Matrix for months now (although Riot hasn't exposed UX to configure them yet, as we were drowning in cross-signing work).
Hmm, that document seems to indicate they're disabled by default in synapse though?
>Note that over every server in the room, only the ones with support for message retention policies will actually remove expired events. This support is currently not enabled by default in Synapse.
https://github.com/matrix-org/synapse/blob/master/docs/messa... has the details.