I think they should keep the high ground advantage to simulate all of the 56,000 to infinity ways that players will improvise tactical advantages in a PnP game. Ideally, combat in DnD is the players constantly thinking of creative ways to maximize their advantages and the DM constantly improvising complications to throw in their path. In a video game, that is simply not possible. The high ground advantage provides a way for players to turn the environment to their advantage with careful positioning.
It also provides an alternative to simply conjuring a fog and hiding in it at the end of every turn, which is currently overpowered due to bonus action stealth, and would be the default tactic in almost all circumstances if high ground didn't provide a sizable bonus.
FInally, this game uses a lot more adaptive lighting and shadows than I have ever seen in a PnP game. I love it. I wish that I had the patience to do this kind of lighting in my games, because it adds a lot both flavorally and tactically. However, it does mean that I'm shooting into the shadows a fairly significant amount of times. Almost as often as it is providing advantage, high ground is just negating the disadvantage from shadows.
I never played the game with backstabbing, but I don't like the sound of this mechanic at all. However, they should re-implement sneak attacks if an ally is threatening (or within five feet, the difference is negligible). A +2 for flanking would be nice but is not necessary. The sneak cones already provide a facing mechanic that is immensely exploitable without an additional flat numerical bonus.
I read through the first few pages of the thread and saw a lot of comments arguing that the advantage from height is overpowered because it's just as good as Foresight, which makes me wonder if the commenters have ever played DnD, indeed if they have ever played BG3. You still get all the same advantages from Foresight for any roll that is not a ranged attack from high ground. If you do find yourself ranged attacking from high ground, then you can do it while casting into shadows or being threatened by an attacker and still get the advantage that would otherwise have been negated. Foresight and most of the other abilities cited are still very good abilities to have.