[wlesieutre]

Another major engine fork is for Star Citizen / Squadron 42.

They've hired several ex-Crytek engineers at the Foundry 42 office in Frankfurt, and among other changes have modified the engine to support a 64 bit data in coordinate system and rendering, multithreaded physics, and independent physics grids for inside the larger ships while they're moving around. Probably other things, but those are the main ones.

[wlesieutre]

This sort of thing is actually pretty common in games; you end up doing some engine customization to fit your needs, and eventually you have to say "We've diverged too far to try and keep following the main updates anymore," and you stick with a customized version of an old release.

Nearly every AAA game is going to fit into that pattern, it's just that the 64 bit coordinates overhaul in Star Citizen is probably a larger and lower level architectural change than what most things bother with.