matrix-doc/proposals/1866-invite-unsupported-version-error-code.md

MSC 1866 - Unsupported Room Version Error Code for Invites

It is currently unspecified what error code should be relayed to clients when they attempt to invite a user on a remote server that does not support the room version.

The proposal is to reuse the M_UNSUPPORTED_ROOM_VERSION error code that is currently returned by the create room API.

Strictly, the error returned by the create room API would mean the local server didn't support the room version, while for the invite API it would mean the remote server didn't. However, there is sufficient overlap that it makes sense to reuse the same error code and rely on the context to differentiate the two cases.