Years ago I remember hearing about something that happened in an MMO where there was a class that was particularly bad got a patch which gave them a useful ability, and the player base then had a conspiracy to never talk about it on the forums so it wouldn't get taken away (as soon as someone was like, hey Power ____ is awesome! they were all like, shhh!)
Trying to remember the details (which MMO, class, etc). I can't remember if I read it here or on rpg.net.

I think that sort of thing has happened a few times. I know in Asheron's Call there were a few "broken" things that were secrets from the devs for months. AC wasn't a class-based MMO, so I don't reckon it's what you're talking about though.

Can you at least recall the game? EQ would be my bet.

Nope pretty blank I can't remember now if it was something that came up in discussion around 4E's errata cycle or 5E playtesting maybe. Probably 4E. Vague memories of warlocks possibly being mentioned, or Fire of Something? Sorry Thanks anyway.

Fairly common.

Players will hit on something good and try to keep it from getting nerfed. That goes back to MUDs even.

Ones I recall.
Something about Rangers in Neverwinter online. Nerfed.
Something about Engineers and Adventurers in Anarchy. Nerfed.
The newby tutorial area in Anarchy was a good place to pick up some early levels if you were patient. Also great for some starter equipment. First the good spots were removed. Then the whole thing. Champions Online had the exact same problem. The tutorial story is now only half what it once was and most of the side quests are gone. The whole mission was removed. But is now back in its diminished state as a training room recreation.
Something about summoner types in Neverwinter Nights? nerfed.
A quest in MechQuest which made getting gold less of a monumental grind. Nerfed to hell.
In fact anything that made any game from Artix less of the hellgrind than they allready were tended to be hoarded secrets as inevitibly the devs would remove it.
and so on.

I don't think any of those were the one I'm specifically thinking of, but thanks

Destruction Warlocks in WoW WotLK? Or was it some Mage build or other?

I do remember something like this in WoW circa 2008-2009.

Though the story also sounds like an urban myth. Players are more likely to hoard info than share it with others and risk the secret getting out. Mainly because these sorts of advantages get shown off to the public sooner or later no matter.

Hell. Going "shhh! Dont talk about this kewl powa!" will just urge someone to brag on the fora that they have the kewl powa or to champion to get it nerfed just to fuck with someone.

I'm far more inclined to believe it happened often than urban myth.

In Asheron's Call, there was a chest that poured out great loot, respawning every half hour, in an otherwise mediocre dungeon. We'd stand in line and take turns looting it, until the devs noticed what was going on, and nerfed it. It took a good month or two, though.

I'm pretty confident nobody wrote the devs and said "hey that chest has unfair loot in it", and I imagine similar happened in other games, many times.