[naniwaduni]

The functional impact of _ENV in Lua 5.2 below the parser level is simply that globals/function environments no longer exist, becoming syntactical sugar for access to a closed-over table. There's a fairly direct (as in, you could write a compiler for it) source-to-source transformation on a 5.2 program (that doesn't use debug or string.dump) where you replace every "global" access with explicit access to _ENV, resulting in a 5.1 with the same semantics as the 5.2 source. This came along with ABI breakage (since PUC then removed all the interfaces that interact with function environments), but at the design level the change only relaxes constraints on interpreter implementation.

5.3 does change up the data model in ways that are incompatible with certain desirable interpreter features (you generally have to choose between 64-bit integers and NaN-boxing), but the incompatibility of 5.2 specifically with efficient interpreter design is at best based on a misunderstanding based on pre-release discussion on the mailing list.

(Real talk though, Lua 5.2 was being floated around the same time period that Mike Pall had committed to LuaJIT 2 as a full rewrite. You can read a bit in between the lines of just the RC dates.)