Stormonu
NeoGrognard
D&D has done "powered" down creatures for ages. Young dragons and juvenile giants are just two cases where such things have existed since 1E.
In The Winter Tapestry (Dungeon #78) that I wrote, it's got a family of Frost giants, with a teenage son, an "8 year old" frost giant and a juvenile white dragon, all with reduced stats. So it's definitely been done before, and it can create some interesting encounters when the party isn't in murderhobo mode.
As others have said, defeat need not mean death - capture, imprisonment and like can be used to give the party a second chance to try again with making it clear that combat isn't going to be the way to victory.
In The Winter Tapestry (Dungeon #78) that I wrote, it's got a family of Frost giants, with a teenage son, an "8 year old" frost giant and a juvenile white dragon, all with reduced stats. So it's definitely been done before, and it can create some interesting encounters when the party isn't in murderhobo mode.
As others have said, defeat need not mean death - capture, imprisonment and like can be used to give the party a second chance to try again with making it clear that combat isn't going to be the way to victory.