curl -sS -X POST 'https://matrix.example.com/_matrix/client/v3/rooms/!abc123:example.com/forget' -H "Authorization: Bearer $ACCESS_TOKEN"

Removes a room the user already left from their account, hiding it from their room list and stopping sync traffic for it. Only works after leaving, and forgotten rooms are re-added if someone invites you back. Pair with leave for a complete exit. The admin equivalent of removing a room for everyone is the delete endpoint.