Consider permitting homeservers to consider leave == forget

[turt2live]

The spec says that users must forget a room on their own, however I'm not really aware of many people who do this intentionally. To assist homeservers in their ability to garbage collect rooms, maybe we should allow them to forget rooms on a user's behalf.

We might want to put restrictions on it (ie: "thou shall not forget a room which has been left in the last 15 days"), but homeservers should get the ability to automatically and arbitrarily forget rooms.

[richvdh]

honestly, I've always considered forgetting rooms that users have left to be a thing that implementations/deployments can choose to do if they wish. I agree that this should be made more explicit though.

[clokep]

Synapse has an option to do this since 1.84.0, see matrix-org/synapse#15224.

[turt2live]

I think this just needs an MSC then, which should be relatively easy to justify given Synapse has had the feature for a little while with presumably great success.