I don't see a reason not to update LuaJIT still. Changes in Lua aren't just version numbers, it should be improving something, meaning that would be missing in LuaJIT.

Isn't it a bit naive to declare that, just because Lua created a new minor version, it should be somehow better? The author of LuaJIT has often written his arguments, including why he disagrees with the changes to the language, why they should have been implemented as libraries instead, that in his view LuaJIT is still more performant and more widely used than PUC Lua, and more.

As for forking, you can try, but I would warn you that one does not simply fork LuaJIT. Required is deep expertise in tracing JIT compilers, in assembly and in many different computer architectures. Nobody was really up to the task when Mike Pall announced that he was searching for a maintainer, before his eventual return.

LuaJIT does have some backported features from newer versions. But Mike Pall -- the mad genius behind LuaJIT -- has made it clear he doesn't agree with many of the changes made in newer versions, hence why it's staying where it's at.

the beauty of open source is there's nothing stopping you! this might be your calling. best of luck