Each new Lua version has breaking changes that are of dubious value to keep on the upgrade treadmill. Something like a Python2->3.
LuaJIT is famously on 5.1 with no signs of moving.
Not moving straight to a more recent version, but still cherry-picking some parts of them. You can see some functionalities from Lua 5.2 and 5.3 are working in Luajit: https://luajit.org/extensions.html