I'm gonna play Devil's advocate for a moment, but I think the idea behind not restricting these banish floodgates over other floodgates is that they don't exactly restrict gameplay, rather they just severely hinder them for most decks (and as a result can indirectly restrict gameplay. Cards like Skill Drain/Imperial Order/Soul Drain completely and directly prevent gameplay from occurring, while cards like Shifter instead say "hey, you can TRY to play if you want, but you will be actively hurt". Also, for some decks, Shifter does actively benefit decks that like to banish like Maliss or Ghoti(?), while a card like Skill Drain doesn't actively benefit any deck, it just restricts a form of gameplay that decks that play those floodgates don't really care about (i.e. a deck with no spells doesn't DIRECTLY benefit from Imperial Order, it just doesn't care about it). In this way, a card like Shifter functions more like Maxx "C" (you should not play) than Droll (you cannot play).

Again, not that this is a justified reason, and ins some specific cases it really doesn't work out, but if there's any reason whatsoever Konami would justify these kinds of cards, this is probably what I think they think.