See https://github.com/matrix-org/matrix-spec-proposals/pull/4228 for details. Since this is tricky to test without server-side support, I have added a basic implementation to the mock server in appiumtests/login-server.py 1. Start appiumtests/login-server.py 2. Start neochat with "--test --ignore-ssl-errors" options 3. Open "Explore Rooms" 4. Search for the exact string "forbidden" 5. See new error message provided by server