- The official server (arcturus) is awful to work with code-wise. But all the decently big private servers uses it because at one point we only had the binaries of it and it worked out of the box. When the source started to leak too, it was easier to continue forward with that thing.
- The "simple version" which I am assuming you refer to is os(i)rose. This was the only thing you would get BEFORE the official server got leaked. It had some momentum simply for being there since roughly 2006. It was based on Brett19's code which at the time was a 14-something teenager. The same brett that now works on a fully modern C++ codebase that is decafemu.
- The Modern C++ version which if I remember is worked by few folks from osrose came out something like 2 years ago. Passing 2015, the momentum for the game is close to none. So yes, no one will even spin that codebase.