TLDR: Do not play battlegrounds if you are level 10-49 and if you are alliance until this is fixed.
This bug still exists, it was reported earlier and not fixed. Supposedly (according to twink communities) this has existed for several months now and blizzard doesn’t want to fix it: Enthralled Val'kyr brought into Bg's
Image proof of the mob in a BG: imgurDOTcom/a/gGrMq5N
The gist is that the same player is still using this in 10-19 bgs. The mob they’re bringing into BGs has 10k hp where the average player HP is 1k-1.5k. It casts slows, stuns and 1shots anyone it targets within range. I had qued 3 games in a row with said player, they did not hesitate to use said mob every time.
The mob is a quest NPC from the western plaguelands, clearly not intended to accompany the player into battlegrounds. The mob is horde-only also, making battlegrounds unsafe if you are alliance. Supposedly this mob is also brought into the 20-29 bracket also, making all BGs in the level 10-49 range completely unsafe to play on if you are alliance.
This is the mob in question from wowhead, as you can see people have already discovered it’s abuse in battlegrounds in the comments section: wowheadDOTcom/npc=45080/enthralled-valkyr