Looks like LuaJIT is catching up, but calling these "syntax extensions" is confusing. Is the intent to hold LuaJIT fixed against some earlier Lua version (I guess 5.1) and adopt newer syntax piecemeal?
I welcome the compound assignment operators. Playdate's version of Lua also has that extension.
LuaJIT is an involuntary fork of 5.1. It already had various extensions that conflicted with the 5.2 implementation of the same features, and Mike Pall made it clear on the mailing list he wasn't going to change how LuaJIT worked.