can’t handle losing 1 piece?
Over the last couple months I have encountered many players who will resign if they are about to go down 1 piece. Is that what “rage quitting” is? How will they get better if they don’t soldier on?
I don't think that qualifies as rage quitting. That is more likely long experience resignation
Sometimes I play my favourite chess when I’m down material, as it frees up the brain a bit; I’m already losing, so there’s no downside to taking risk.
That said, there are levels of chess and otherwise stale positions where the loss of a piece is simply too much to compensate for. Particularly in slower time controls.
So resignation is now beyond sportsmanship

I think back when I was like (400) - (700) I would resign when I am down two pawns or a piece or a exchange.
Now I know how to create winning chances, but weaker players might not know that or can’t do it that well. Winning a winning position is for weaker players harder, yes. But fighting back fron a loosing one also.
Therefore I resigned a lot back then.
Did I rage quit or tilted? Yes, sometimes. But playing on simply seemed highly irrational to me back then.
A number of good comments here. Long age I knew class C players who thought an one pawn advantage was winning in a middle game. Just didn't understand how to complicate positions and just traded to endgame. I think MrChatty makes a good point in that resigning can't really be criticized. Resigning can't be considered rude.
I just want to make sure I am understanding the OP- it's rage-quitting and poor sportsmanship to resign a game if one is down only a minor piece?
Perhaps it's about picking one's battles and conserving energy. Who knows what the opponent is thinking? But why jump to the rage conclusion?