The old forward was simply bugged, everyone expected it to catch all disconnects, because that's what it was supposed to do. Even if such a case exists that would strictly warrant the use of the old unreliable behavior (it doesn't), the fact that the forward is deprecated, means it would be strongly discouraged anyway.
Doing shit like this...
PHP Code:
#if !defined client_disconnected
#define client_disconnected client_disconnect
#endif
...just so you don't accidentally leave a ghost function that would never get called in an older AMXX version, is a retarded design. It serves the same purpose as having only 1 forward from within AMXX, as it should be.