Mob tagging allowed ?

Hey
I was trying to level up some alt and was surprised that my xp gains on the level 5 was low... kept going until level 17 where i would gain like around 100 xp per mob, which was with rested xp and thats super low with x5 xp.
I was using a level 70 to fasten the proccess, basicly tagging the mob with my new alt and killing with my main. Therefore i noticed that i would gain way more (300+ instead of 100) per kill where i would only use my lvl17.
At this point i suspected a bug, so went on the bugtracker and found https://www.warmane.com/bugtracker/report/74251
Report is dismissed without explanation. I guess we are not allowed to mob tag in any case ? (I remember doing this a few month ago thought..)

it is recent fix to avoid that kind of power levelling

I do not know if it is a custom change or it was like that on Retail