fwiw, the linked MSC https://github.com/matrix-org/matrix-doc/pull/1915 https://github.com/matrix-org/matrix-doc/pull/1915 solves the problem, is merged, and is

fwiw, the linked MSC https://github.com/matrix-org/matrix-doc/pull/1915 https://github.com/matrix-org/matrix-doc/pull/1915 solves the problem, is merged, and is implemented in https://github.com/matrix-org/synapse/pull/4982 https://github.com/matrix-org/synapse/pull/4982. Yes, it would have been great to have done this sooner, but unfortunately fixing a bug which only impacts deactivating accounts when using custom identity servers took a while to get to the top of the todo list.