Its fully complete. Because it was reversed engineered function by function, it was never really not feature complete, since it would still call out to existing code in the binary (thought there were some bugs). They've since added a ton of new features not in the original game (eg multiplayer networked mode), native OS X/Linux support, etc.

I contributed a little bit back in 2014/2015 and it was a lot of fun. Because it is almost all hand-written x86 ASM (which is insane), it was actually not too hard to reason about, certainly far easier than what Clang/GCC will spit out.