I thought it was a British thing. As an American, I’ve never met a John going by Jack (at least, I don’t think I have).

On the other hand, C.S. Lewis apparently went by Jack, which isn’t a common nickname for Clive or Staples. But people generally don’t argue over nicknames. And they may not even realize a nickname actually is a nickname; it’s not common for people to ask for your legal ID, after all.

People still talk about Jack Kennedy, the assassinated US president.

Apparently Jack is a generic name, too, for any small boy. So inns would employ a boot-Jack to help guests get their boots off, later supplanted by a gadget.

It’s pretty American - I know a boy named John after his father John, who has been called Jack all his life.

Jack is indeed a common nickname for John.

And not just in America. It's common in Australia too, and apparently it dates back a long way in England. Jack has become a lot more common as an actual given name over the past few decades, though.