There's absolutely no proof one way or another if Asmodeus is going to stay a god, or not. In 4e, he was considered one, but even then, he was never part of the human standard pantheon.
I'm personally leaning towards yes, still a god, but vastly reduced in importance. They're likely going to be doing the same thing with Asmodeus they did with the succubus. Are they devils or demons? Answer - make it so that its both. Why? 1) We've seen hints about ashmedai cultists in FR. 2) 5e prides itself on "the everyman edition." They're going to keep enough to play any edition, and keeping Asmodeus as a god allows people that like him to continue using him, but doesn't hurt anyone that doesn't want to use him. 3) Asmodeus has had the option of being treated as a god in any number of source materials.