I feel like Larian seem to have a few main options with ways to go with these two (High ground advantage and backstab advantage)
1) Leave it as is and diminish/invalidate 56+ spells/abilities, some of which are core parts of a class/subclass/spell list - Terrible idea as then they have to look at adjusting all these other abilities to compensate or just make them feel almost worthless. - This isn't counting even higher level spells abilities, just the ones the OP mentioned, it's an insane development resource cost to try to fix the balancing problems introduced from keeping it this way.
2) Modify it so they don't give advantage but still give some benefit. A +2 bonus from high ground and -2 malus from low ground emulates 5e half-cover decently well and a +2 Backstab bonus seems to be a common homebrew instead of advantage for flanking. - Not a bad idea, still gives people a feeling that using verticality/backstabbing has a tactical impact - Still allows for all the 56+ advantage/disadvantage granting abilities/spells/features to have proper impact and not feel like wasted resources
3) Remove it completely - Unfortunately makes the emphasis of verticality in game not feel as impactful - Most in line with how 5e Tabletop works
Personally I would like them to implement both 2 and 3 and have it as an option but I would settle for option 2.
If both 2/3 were options people could choose from, then people that want a more pure 5e implementation can have their way, and those that don't care about 5e specifics still get the option to make backstab/highground impactful.