Fractal calls them Historical rooms and they are rooms that I left on other apps.
I thought that when you leave a room on e.g. Element it’ll send a deletion request to the server and then to other servers if applicable. But I can see all historical rooms and the data is still there. It even shows when I left the room.
How scary is that?
What you see is the historic version, you should not see any messages after you leave.
This can be shown because the room is not immediatly deleted, it just has 0 members and should be purged later when there are free resources on the server, as deletion and clearing up the database takes some work. Actually most used Matrix server, Synapse, is also very bad at clearing after itself so most admins currently need to write scripts to clear those rooms.
Empty rooms makes sense from the protocol perspective, as they can be used to store data for something other than chat. Still we should have more ways to tweak the behavior.
Last event I saw in those rooms was that I left.
There were rooms older than a year :/