In a nutshell, this prevents an error being raised when UNLISTEN is issued during recovery. The operation is a no-op (since LISTEN is still disallowed). This logic here is that some clients (namely Npgsql) issue UNLISTEN * to clear connection state (in the connection pool), but this needlessly breaks when the backend is in recovery.
On a related note, there currently doesn't seem to be a good way for clients to know whether the backend is in recovery. As a backend can come out of recovery at any point, perhaps an asynchronous ParameterStatus announcing this state change could be useful.
Hopefully this also qualifies for backporting to earlier version branches.
