Floodgate works on monsters that cannot be targeted -like Cocytus and Sabre Dancer-, Fiendish doesn't. So I doubt it takes Floodgate's place but almost as good as it
"Any top tier deck still using normal monster?"
Not top tier, but still widely used: DM, Blue Eyes.
I don't know why you mentioned about normal monster, since Fiendish Chain cannot be used to them.
you dont need fienish floodgate stops most of it its just another abomination cards which doesnt let the opp play.at least you could attack fkn unfair board locker sh i t co c k card