Homebrew, whether big or small, is when the DM adds something. Throughout the books it says the DM can follow the rule or change it, that is one of those instances. A DM saying you have advantage on hitting a target because you have the highground is a minor instance of homebrew, compared to a major instance like a DM making a custom Monster to attack you with. However, Homebrew needs to be careful as it can break the balance of the game, as seen with much of the homebrew in BG3. Highground Advantage is just one instance of it going a bit too far and needing toned back. Yes it makes sense for ranged attacks to be easier when one has the highground, but making Advantage granted purely by that invalidates a whole slew of advantage granting abilities such as upcoming Barbarian rage and Find Familiar Help Action. Abilities that 5e was balanced around.
