WoW is keeping factions but (in specific circumstances - namely instanced, non-queued PvE/cooperative content) experimenting with allowing players from different factions to play together.

Because this only works for manually formed groups, it will be purely opt-in. You can't queue up for a random dungeon or Raid Finder and find horde and alliance dropped in together, it has to be a conscious choice by the group leader.

The big benefit here is that a manually-formed groups are necessary for top-end PvE content, things like Mythic+ dungeons and Heroic/Mythic raiding that require communication and coordination. Over time, players who want to participate in that content in most most regions (especially North America, EU, and China) have been essentially forced to play Horde or else find themselves with few to no prospects for organized groups. (In Oceania it's the opposite - the majority of mid-high to high-end players are Alliance.)

This is a positive change, but the big hurdle they have yet to clear are guilds - which are still locked to one faction. So your guild can play with Alliance players, but you can't actually invite them to your guild to benefit from the social structure that exists outside of that content. Communities, or external platforms like Discord, can be used to coordinate instead, but both have their drawbacks as well.

Still, as mentioned, it is a positive first step overall.