I thought about this too tho. Some of the rank 4 spells look amazing and i’ll be sad if I can’t use them anymore in 9.0
My guess is that they’ll add them as toys or spells in our spellbook that have no effect anymore whatsoever, just the visual
Even if they don’t remove the way you can get it, you will still not use it anymore as they just enhance the essence visual effect and not your characters effect.
You will lose heart of garbage in 9.0 so you cannot focused azerite garbage in 9.0 and thus have no vision effect anymore.
so actually it doesn’t matter if they remove it or not