Honestly I think the issue is that this ABI breakage is exactly the kind of issue that's easy to slip by. It's not obvious even reading the old discussions of the bug resetthread is fixing that the fix is an ABI break and the description of the lua numbering scheme is 'Different releases of the same version ... are binary compatible (ABI compatible)'. I can see why nobody would have tried the sort of tests that could expose this.
In any event this is now a pretty major bug with 5.4.5 that needs to be addressed somehow. Possibly with a relatively rapid release of 5.4.6 that just fixes this bug by adding a new lua_resetthread2 and reverting the ABI break.
-Alex