In BfA I think it was they changed them later to try to balance factions per-zone, but there are overflow shards where the more populated faction will end up in shards by themselves.
As fewer people bothered to turn War Mode on at all, I think most zones are probably fairly empty now for everyone, but they did have a system in place where it will balance factions to some extent when there are enough people in the same zone to do so.
They actually experimented with doing this with layering in Anniversary Classic as well and compared it to what they do in War Mode in retail:
Hey folks,
I wanted to drop by and let you know about a new, slightly experimental, feature that is coming with patch 1.15.5, and the Anniversary realms launching on Thursday.
As many of you will know, with Season of Discovery we implemented a system for PvP ruleset realms that restrictions faction creation when one faction becomes dominant. This system worked fantastically for keeping one faction from snowballing and has maintained a healthy balance throughout Season of Discovery. We’ve already announced that this feature will be enabled for the Anniversary PvP realm.
But, one of the consistent pieces of feedback was that a faction balanced realm does not lead to a faction balanced world due to how the layering system handles player load.
With 1.15.5, PvP realms will attempt to balance faction at both the layer and the specific zone level. What this means is that if one faction exceeds our balance threshold, any additional players attempting to enter that same zone will instead transfer to another layer.
The system will work much like our Warmode system in The War Within, where we balance as many copies of the zone as possible and eventually any overflow of one dominant faction will end up on a one sided zone by themselves.
As part of this change, we are also going to heavily restrict players ability to “layer hop” on PvP realms, you will face a much longer cooldown before the system will allow you to change layer again that will also ramp up with each successive layer change. The idea behind this is try to maintain these layers faction balance and discourage a mass influx of multiple raids of one faction trying to flood a specific area.
Whenever we make a change to these sort of systems there are a ton of unknowns around quite how it will behave once real humans get involved. We will be closely monitoring how this all feels in-game and will make changes or outright disable the system if it ultimately leads to a negative play experience.
We’re excited to see how this plays out and the carnage it may lead to. One final warning however, please be very confident that you want to engage in Player vs Player combat in Classic Era (including the Anniversary realms) as this time around, there will be no escaping the other faction by switching to a layer that is dominated by your faction. If you sign up for PvP this time, you’re signing up for everything that entails.
Whether you roll Alliance or Horde, good luck, we’ll see you on the battlefield.