there's one card in the mythos deck that removes all elder sign tokens from the board. If a location has been sealed and the token is removed via the card, is the location still counted as sealed for the purposes of gates appearing?

Nope. That card wouldn't be much of a threat otherwise!

The card of which you speak is a Rumor ... Good Works Undone , so you do have some control over the fate of your Investigators ...Pass that Rumor !