matrix-org
Decide whether we want to support sending membership events for users that do not exist
Description
It is currently possible to send membership events (invite, kick, etc.) about users that are not known to the sending server. Typically this has not been a problem, but can arise in confusing behaviour such as when a user registers a new account and finding that they have existing invites (https://github.com/matrix-org/synapse/issues/7898).
It can also result in oddities such as being able to kick users from a room that do not exist (https://github.com/matrix-org/synapse/issues/2959).
However it can be advantageous to ban a user from a room even if the homeserver they're on claims they do not exist, therefore we may need to think about this on a per-membership event type basis.
We'd also need to be careful about appservice users, which can be created on the fly
