We've got yet another idea for how to implement alias semantics, documented in https://docs.google.com/document/d/1NNDkobiFLeUkJtyj0H6qvKIedgvIkZnFKo78-03cGEk and MSC2432.

The task breakdown for now is:

• Update {PUT, DELETE} /_matrix/client/r0/directory/room/{roomAlias} to stop sending m.room.aliases events. Update DELETE /_matrix/client/r0/directory/room/{roomAlias} to check the alt_aliases in m.room.canonical_alias.
• Update POST /_matrix/client/r0/rooms/{roomId}/upgrade API to stop copying room aliases (and thus to stop m.room.aliases events). Update POST /_matrix/client/r0/createRoom to stop sending m.room.aliases events.
• Add a new GET /_matrix/client/r0/rooms/{roomId}/aliases API, which returns the list of aliases which are currently available on the local server. Accessible by any member of the room.
• Update /_matrix/client/r0/publicRooms to stop sending a list of aliases.
• Update Synapse to stop considering m.room.aliases events for room names
• Update {PUT, DELETE} /_matrix/client/r0/directory/list/room/{roomId} to check the sender's PLs against that required for m.room.canonical_alias instead of m.room.aliases
• Update PUT /_matrix/clent/r0/room/{roomId}/state to do validation on the alt_aliases in m.room.canonical_alias
• Update DELETE /_matrix/client/r0/directory/room/{roomAlias} to allow it to be used by local users with PLs sufficient to send m.room.canonical_alias events.