Olaf the Stout
Hero
So the concern is you're getting into unintentional-house-rule territory, and the players may be getting used to getting advantage easily, in that specific way? Valid. If "we've been playing it wrong," gets you past that, by all means, go there.
More subtly (I'm assuming you're the DM here), you could set some fights on more gently sloping terrain, and when they move to the 'slightly' higher ground for advantage say 'no, it's not as much of a height difference this time,' a few variations on that, and they'll get the idea they can't count on it every time they stand on a soap box.
...apologies if you weren't looking for advice, just rules confirmation, which, clearly you've gotten at this point...
Nope, I'm a player (although I'm usually the DM of the group). It's our first time playing 5E (4 sessions in) and I'm playing a Rogue, so I'm obviously interested in getting advantage wherever possible.
So I was looking at different ways to get advantage, which is when I couldn't find where it stated about higher ground.
I'm not entirely sure how it happened, but we've been playing that higher ground grants advantage for prett much all 4 sessions (I could have possibly been the one that incorrectly stated to the group it was a rule and they just trusted I was right).
Given that my PC benefits the most from the rule (although the DM has definitely also used it to get advantage), I figure it's best to let the DM know about it so we can change how we rule on it going forward (maybe only granting advantage on melee attacks or for specific situations where it seems merited).