It is fairly stable in terms of syntax. Just that features like Unicode support etc. tend to be difficult to implement without deep changes. For OS dev, look into Node9 and https://www.lua.org/wshop13/Cormack.pdf

There have been quite a few attempts at building OSes with Lua as main system/userland language. There are very few Lua implementations that can bootstrap so you still need a bit of non-Lua code as glue for the interpreter bits if not for juggling pointers.