LuaJIT "forked" from Lua during the 5.1 days. There are still some features from 5.1 that Mike (intentionally) didn't port to LuaJIT.

My understanding is that LuaJIT isn't really under active development anymore.

It does seem like it would be hard to add the 5.3 features to LuaJIT without compromising on efficiency, but given how clever Mike Pall is, I'd suspect it can be done.

It is under active development. There is the start of an arm64 port recently for example. Mike did take a break to do other stuff for a bit it seems, but its not stopped.

Ah, the note at the top of http://luajit.org/sponsors.html had made me think he was not.