"Note: this provides only partial compatibility with Lua 5.2 at the language and Lua library level. moonjit is API+ABI-compatible with Lua 5.1, which prevents implementing features that would otherwise break the Lua/C API and ABI (e.g. _ENV)."
Some internal language changes in 5.3, like the new integer type, would require deep surgery in the JIT. This also seems to be the main (but not only) reason why Mike Pall doesn't have plans to ever support Lua 5.3. (https://www.reddit.com/r/lua/comments/2zutj8/mike_pall_luaji...)
LuaJIT had a beta release 3 years ago and hasn't been updated since.
The last commit is from 5 days ago.