I would argue that the use of the accusative case with the verb
to be (and other phrases like
me, too, although not with questions like
who is he?,
where am I?, etc.) is entirely correct, even though grammatically it may sound weird and wrong, particularly to non-native speakers. Like PitaBread said, languages are constantly evolving, and I feel like
it is me/him/them/her has long since gotten to the point where it is not a mistake at all, but rather an accepted, even preferred exception to the rules. I don't know how or when this habit originated, but I compare it to those old, established phrases that don't always follow standard grammar rules, either, such as
long time no see.
I'm speaking as someone who is very much an obsessive grammar police and who freaks out at mistakes as minor as a missing comma. Anyone who knows me can confirm that, I'm sure.

Generally, the only times you will ever hear a native speaker say "It is I" nowadays are when you're talking to some (but not all) grammar buffs or you are in a very formal situation, and even then, I think most people still say "me."
Using pronouns in the accusative case as the subject with other verbs, though, like in the sentence "John and
me went to the store," is still considered incorrect and should be avoided, even though native speakers often speak like that in casual speech. If you want to sound like an educated person, you should say, "John ('He') and
I went to the store."