In 5e the best strat would be to bottle neck everything at a choke like a door with melee characters and keep ranged/caster behind. Mobility would be static because there wouldn't be a huge reason to flank depending on if you wanted to include the flanking rules or not as mentioned above, so it becomes a face to face slug fest.

Ground effects and positional advantages force mobility versus static formations. Typically it was too the defenders advantage to take the high ground and defend from a wall or a cliff, otherwise traveling through a mountain pass with threats of archers on the cliff ledges above you is not as dangerous or necessary beyond flare, but they could have easily just been on the ground standing behind 3 or 4 big bruisers.

We are in a three dimensional playing field, why not use more than two of those?

Is it too strong? Maybe? At early stages it is difficult to land attacks from the low ground. Advantage/disadvantage is rolling two dice and picking the best/worst of the two, correct? So really there is no modifier making you roll better or worse, you still have to roll the correct DC with your current stats. The closer to the DC you are with your hit modifiers, the easier it is to overcome disadvantage. My hunter/ranger with +2 in archery and +4 in dex now has a +6 in chance to hit. If the AC of the target is at 12? (I don't remember what the average AC you run into in the game is at the moment) I only need to roll a 6 to land a hit. Out of 2 dice rolls, a 6 or higher is quite doable and I find in game I land hits from low ground pretty consistently.

My other characters who don't have as many hit modifiers struggle to hit from the ground as they were meant for melee or spell casting. Str and Int/Wis/Cha characters don't seem to get better at elevation fights over time as str does not help at ranged attacks and Int/Wis/Cha only applies to saving roll modifiers for resistances on spells that use them. Otherwise they use ranged chance to hit rolls via Dex (need to confirm this as I am word vomiting based on my understanding of the combat mechanics).

So spells and abilities that use spell DC rolls would be more reliable ways to hit targets of elevation with a caster, as you can level/equip gear to increase the spell DC and increase your primary stat to add modifiers to it. Currently in bg3 there is only the ogre headband that increases int, and that is only to 18, so you can't get +1/+2 gear for casters to help eek it up higher.

The other suggestion I have seen is having elevation give adjustments into AC. That would work I guess, but theoretically it makes it harder to land a hit as the roll you need to land on the dice is now higher, versus you get 2 chances at the rolling the dice to hit a lower number. Both result in less chances to hit, but one could put a target completely out of your chance to hit range while the other makes you roll twice and take the lower/higher roll.

If the target AC is 20, and elevation gives them a +2 to AC, you now have to roll a 22 with modifiers. If your modifiers are only +2, the highest you could ever roll is without landing a critical hit is 19. Add modifier and you roll a 21. So now you have to crit regardless and you only have 1 of 20 chances to land. With advantage on elevation, you could succeed with 18, 19, and 20 providing 3 of 20 chances to land a hit, but then you have to roll 2 dice. The odds are the same both times, and subsequent rolls are left to rng. We can argue gamblers fallacy or probability over time, but that can get into a rabbit hole.

So I guess what I am getting at is, this combat is different, sure. Is it inherently worse? Subjective. I would say all my years playing "fight in the doorway" in older D&D games was tedious and a yawn fest, but one of the best ways to protect my squishy ranged characters. Is that immersion breaking? Yeah to me, every fight I run to a door or a choke and my characters just cower behind the big hulking warrior as they do almost nothing.

One system makes you hide behind a frontline and play very 18th century military tactics "hold the line! Stand in a square!". The other system makes you chase the high ground that way Anakin will never be able to beat you.