X_LIVE_CLIENT_MISSING
Means
retired in 21.0.0 (no LiveClient to register; see X_REALTIME_UNINSTALLED): a realtime hook ran in a browser with no LiveClient registered
Typical cause
registered, thrown by nothing since 21.0.0. There is no LiveClient to register: the island bootstrap installs realtime, and a hook in a bundle that never did is X_REALTIME_UNINSTALLED
Fix
delete any branch matching this code, because it never runs. For the failure it used to name, see X_REALTIME_UNINSTALLED