Have any previous DnD video games stuck completely to the rules? I'm not aware of any. I'm not sure how many DnD TT player groups stick to the rules either.
Even though the game trades on DnD 5e and Baldur's Gate/FR as hooks, I don't really expect it to completely correlate with either. Regardless of how strongly you feel about the mathematical integrity of 5e rules, I think that is probably less important to the game's developers than the play experience matching what their core audience would expect.
The central demographic of video games players likes high quality A/V, likes things that explode, likes Marvel/DC like super-heroics, etc. The game is designed mainly for that audience, and probably needs to be for financial reasons.
I don't really see Larian changing the core game design prior to release. Neither should they, since those that have purchased EA could see exactly what they were buying from the early gameplay videos; it has not changed.
As with most of the more fundamental game changes that different players want, complete 5e accuracy is more likely as a post-release optional mode or from the mod community.
I wouldn't call "high ground giving advantage" a core game design. It's a mechanic. And there is a difference between "not sticking completely to the rules" and making adjustments that significantly throw off the balance of the system. Getting advantage is one of the most significant things in 5e: it typically cost a spell or ability (see the 9th!!! level spell that grants advantage for the day), and importantly it does not stack.
A very simple change that would fix a lot of problems is giving height/lowground a +4/-4 bonus instead of advantage. Mechanically, this is the ~same as advantage. BUT this method wouldn't invalidate the usage of the dozens of spells, abilities, feats in 5e that grant advantage.
Similar for backstabbing: it is in no way part of Larian's "core game design" and invalidates a lot of spells/abilities that grant advantage.
