I agree that getting advantage on all high ground ranged attacks is powerful. But,
1.) By choosing this feat, you're forgoing the likely +1 to AC, +1 to ability checks, +1 to Dex ST, and +1 damage that increasing Dex +2 would get you
2.) My idea was that this feat would replace the base (+0/+1/+2) gotten by other characters on high ground, so it's only a net ~15-20% buff to hit instead of a 25%
- 2b) Thus it wouldn't stack with other sources of advantage, whereas normal characters would still benefit from it and get the (+0/+1/+2)
3.) You'd only get the benefit of the feat when you have high ground. Which is often, but not always. Whereas +2 Dex applies to all attacks
All the above serve to make the feat less overwhelmingly powerful. But I admit it still may be too OP.

D&D 5e base rules is not designed to account for Facing, but the DMG does have Optional Facing Rules. Which at the very least means that it isn't crazy to homebrew facing rules (or adapting the DMG optional rules). The main problem, as you state in your post, is that BG3's "Backstab Advantage" is gotten for free. A feat cost may be too small of a cost, but it's more than the cost in BG3 right now.

The more I think about it, the more I like "Backstab Advantage" being replaced with some kind of "Backstab bonus to damage," probably with a feat cost.