Comment by Heliodex

I wouldn't be surprised if their plan is to keep compatible with 5.1 and adopt newer features where feasible/compatible. Luau, another language with the explicit goal of extending Lua 5.1 in a compatible way, has a section in their documentation listing all newer Lua features and detailing why they chose to or not to adopt them <https://luau.org/compatibility/>. The needs of Luau and LuaJIT are different though the reasoning is nonetheless fascinating.