This is effectively settled. Every client worth using has standardized on certain extensions, and there are server capability testers for it [0].
There are still a LOT of things very much up in the air, such as message deletion, which work inconsistently from client to client, but message history is not it.
There are still a LOT of things very much up in the air, such as message deletion, which work inconsistently from client to client, but message history is not it.
[0] https://compliance.conversations.im/