It's still alive, to some extent. Priit Kasesalu (of Skyroads, Kazaa and Skype fame) wrote an alternative client back in 2001: Continuum. Some of the bigger servers (EG, TW) are still around. Many of the smaller ones like SFSS have long since perished.
Subspace Continuum is an anti-hacking fork released in 2001 (Subspace was released in '95) and is now the only official version of the game.