It still is. There's been attempts at porting the game to modern engines (Space Station 14 having the most progress), but SS13 is still alive and well.

/tg/station's github is very interesting, too. About a dozen pull requests every week adding random features and bug fixes from random contributors. The game is constantly in flux, but they manage to keep everything feeling the same.

https://github.com/tgstation/tgstation