Fortnite!
When it got popular around 2018, I got really into it and saw the progression of it getting "better"!
EPIC, the company who made fortnite, started to listen to gamers and it ruined the game. Game is pretty hard, it's not just a shooter game, it has building mechanics - you can build walls to protect yourself... an average good player (https://youtu.be/ajM4OifohOk?t=42), (the really good players are playing at 240hz and you can't even see all the moves in 60hz youtube video.)
I wanted a practice mode, they made it.
I wanted they'd give us unlimited resources to practice, they made it so. I wished they'd give us a way to edit map, they did.... and more.
Game mechanics are hard so it makes sense to have a way for players to practice.
Every time me and my friends cheered, but we couldn't see far ahead.
Game went from fun/somewhat challenging/social hangout time to toxic/grifting type of place. Where only people who spent 5+ hours, not playing, practicing could even survive. To have fun in a game you need to be at least little good. But practice mode and full freedom in it made it so that the only way to have fun was to live a unfulfilling life overall as you'd be dedicating every walking hour on practice.
The whole culture within the game changed, before if you got challenged, you would accept it openly and fight and have good time, after "improvements" people would hide and sit in corners for 15 minutes at a time, only to die. Because now fighting meant you could run into someone who has been practicing 8+ hours daily and has no life.
In 1 year the difference between good and bad player became so wide they had to introduce tiered matching. It tracks how you are doing and if you keep doing better you are put into lobbies consisting of generally better players. This made even worst, if you do start to win, then you get placed with people who 100/100 no-life practice mode habitants - the more you played the less fun it'd get.
A/B testing would have always given a signal that things are getting better, up until a year later, when everything started to go down hill. i.e. Practice mode = engagement++++ week over week!