We last saw Surtur in a knockdown drag-out battle with Cate Blanchett’s Hela at the end of Thor: Ragnarok.
The battle ended in the complete destruction of Asgard, and the seeming destruction of both combatants as well.
But this is a comic book movie after all, so it’s not like reasoning can’t be contrived that would allow one or both characters to return in the next Thor movie.