Nil doesn't exist as a value in Lua, because a variable or a field that don't exist or are unassigned are indistinguishable from a nil value, since they all return nil when evaluating. In particular, this leads to the situation that you can't have a table where some of the fields are assigned nil as the value, because those fields effectively don't exist.

the key does stop existing, insofar as referring to any name that is not defined in lua results in nil. which includes variables, since they are defined in a global table. i don't actually know what happens internally.