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Room IDs being splittable into localpart and servername does not have much inherent value and there are proposals like MSC4051¹ that propose changing the format. Relaxing the rules makes Ruma forwards-compatible with those proposals. The server_name accessor is kept because it is used by at least one downstream, but is updated to return an `Option`. ¹ https://github.com/matrix-org/matrix-spec-proposals/pull/4051