Root
 

Players are currently able to root more than 4 targets.

This is not classic, only as far as my memory.

I am unable to find any substantiating evidence for or against, is anyone else able to?

ah yes the rule of 4 was very powerful in classic. i recall at one point, when you had more than 4 mobs meleeing you, you could back up and then only 4 would get back in melee range. the rest would stand there broken-style, waiting for their turn to engage.

Fixed before or right after Kunark if I remember.

This is how I remember it as well. The rule of 4 also extended to charmed pets not being able to keep aggro if 4 PCs were on the hatelist, iirc.