[Arathorn]

What are you thinking of here? Synapse has supported purging room history since 2016: https://github.com/matrix-org/synapse/pull/911, and configurable data retention since 2019: https://github.com/matrix-org/synapse/pull/5815.

Meanwhile, Matrix has never needed the full room history to be synchronised - when a server joins a room, it typically only grabs the last 20 messages (with other history pulled in demand when the client scrolls up). (It does needs to grab all the key-value state about the room, although these days that happens gradually in the background).

If you're wondering why Matrix implementations are often greedy on disk space, it's because they typically cache the key-value state aggressively (storing a snapshot of it for the room on a regular basis). However, that's just an implementation quirk; folks could absolutely come up with fancier datastructures to store it more efficiently; it's just not got to the top of anyone's todo list yet - things like performance and UX are considered much more important than disk usage right now.