Archived state for a room is not well defined, and state delta requirement seems unimplemented and cumbersome for clients

Nobody has claimed this yet.

**Link to problem area**:
https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-sync

**Issue**
`invite_state` for invited room says that:
> If the client joins the room then the current state will be given as a delta against the archived state not the invite_state.

A room is apparently in archived state when it has membership "leave" but /forgot hasn't been called on it yet (which only the client that calls /forget can known about fwiw). This doesn't seem to be explicitly explained anywhere in the spec.

Also, are server implementations actually serving this delta? It's somewhat annoying for clients to implement keeping state around for **any** left room, given that they can't know if any other device has called /forget on it.

It seems like an uncommon case to optimize, and was probably introduced before lazy loading members was a thing. If it is not relevant anymore, perhaps it can be removed?

Start with the linked client-server specification section for GET /sync, focusing on invite_state, archived rooms, and state deltas. Review the four existing comments before deciding whether the behavior needs clearer documentation or removal. Done means the archived-state semantics and the expected client/server behavior are unambiguous.

Written by the indexing model from the issue text.