synapse secretly uses body of `/join` request as the join event content

[richvdh]

This is totally unspecced, but it turns out that synapse uses the body of a /_matrix/client/r0/join/{roomIdOrAlias} request as the content of the join event.

Apart from being inconsistent with the spec, it also means that there is nowhere to put other options such as server_name (see https://github.com/matrix-org/matrix-doc/issues/2864).

(If there's a need for such a thing, it should be in a content key)

[richvdh]

I'd vote pretty hard for just dropping this functionality. If anyone is relying on it they need to MSC a proper spec for it.

[richvdh]

MSC2513 seems to be an MSC to add it