Lua changed behavior for this specific event. I considered the change
minor enough that it would not need to go through deprecation, but
someone hit it immediately and ask on the -dev channel.
It's probably better to restore the behavior. But mark it as deprecated,
since it's problematic (mismatch with the C API). Unfortunately, no
automatic warning is possible. (Or maybe it is, by playing sophisticated
Lua tricks such as setting a metatable and overriding indexing, but
let's not.)