The way NPC’s react to Fear is definitely different from Vanilla. Instead of ignoring the feared target as if it were CC’d, they continue to attack their current target even if that target is feared.
In Vanilla, anytime a PC was feared, NPCs would ignore them as a target and go after only unfeared players. This had a tremendous impact in how Onyxia + Nefarian tactics worked. If the Main Tank got feared, the dragons would immediately target any player that was not feared and highest on threat.
If you didn’t have Fear Ward, this made stance dancing and using berserker rage the biggest part of tanking in the fight. In old videos you can see onyxia change targets then immediately snap back to the tank once the fear was broken.
For horde raids this required two tanks since the fear cooldown was actually faster than your berserker rage cooldown and tanks had to communicate and alternate their fear breaks.
This is the primary reason druid tanks weren’t a thing since they couldn’t break the fear mechanic and onyxia / nef would often turn on the raid and cleave/breathe. Tremor totem was also notoriously unreliable since there could be a few seconds when the MT was still being feared and in that interval players could get one shot.
In Classic, both dragons just chase the tank around while he/she is feared making the mechanic trivial to deal with. Most tanks don’t even bother breaking the fear and stance dancing isn’t something discussed by most raids.
The fear mechanic changed in BC patch 2.3.0
World environment - Creature AI has been changed to no longer prioritize attacking unfeared targets over feared targets.
This should be reverted which would make those encounters true to their classic form.