Since no one else bothered to answer you I’ll link some threads which answer the question.
Testing Same-Faction Battlegrounds in Burning Crusade Classic - WoW Classic / Wrath of the Lich King Classic Discussion - World of Warcraft Forums (blizzard.com)
Burning Crusade Classic Same-faction Battlegrounds - WoW Classic / Wrath of the Lich King Classic Discussion - World of Warcraft Forums (blizzard.com)
I was never a fan and made a thread asking for an opt-out but ultimately didn’t quit the game over this but did stop doing BGs over it because frankly there is “something about” WoW factions that I like and if that’s not a thing then well… there are over 100 other faction free games that do PVP FAR better. So I just moved on to those.