What does "limited edition" mean on the Might & Magic 6-Pack? Will they eventually remove it?

No, it means that they are selling a digital version of what was the boxed copy of the 6-pack, which did originally include the "Limited Edition" so, the GOG version does too.

Sometimes you see "Gold" or "Complete" often this will mean "includes expansions" or the like.

But as Orryyrro said, this is like Broken Sword: Director's Cut, it's that version of the game, perhaps updated or with the gameplay tweaked a bit, or just the content that was in that version of the boxed copy (in a previous life, when we had game boxes:) ).