I wouldn't say this affected just one class specifically as it was a blanket nerf to quite a few but, the great mass killing nerf of February 2014. It completely changed the PLing scene. Headshot, assassinate, beams, swarming. People had to adjust their playstyle or move on. I would wager a guess that a lot of classes made just for those reasons stopped getting played.
Another smaller one was when they changed Bard dicho so that a bard wasn't an infinite mana pump that could let casters run at full burn almost indefinitely. Lots of bards were made and leveled explicitly use that ability
RIP Disconcerting nerf to berserkers. Systematic destruction of mages and their pets between TDS and ROS. The dark years for paladins between COTF and ROS. Any of a dozen various enchanter nerfs.
Worst ever was a cleric heal agro bump around Kunark or Velious. I mean, real early. Casting Complete Heal was a death sentence for clerics and back then there were no AA so all warriors had was taunt. Clerics quit in huge numbers, the game was in an uproar. SOE blinked and dialed back the healing agro. TSS era. Utter ruination of enchanters. You had clarity potions which killed their best buff, their DPS stunk and everything in that expansion was immune to mez and charm. I remember numerous high end raiding guilds going on The Runes, an enchanter board, begging their enchanters to come back and offering to pay the server transfer fee for anyone who would make the jump. TSS killed enchanters for years.. Right now, for whatever reason, it's shamans. I can't think of a guild that has enough. Bards too.
Monks about 13 or 14 years ago..changes in their ability to tank made them into box dps vs. box\healer box(shaman).....
I remember at least in the circles i ran. The Ranger headshot nerf had more than a few either quit of re roll