At a lot of tables this would violate the social contract. Many tables want PCs to make sense within the game.
OK. So you're insinuating that this particular concept would not make sense within the game, thus violating the social contract?
OK.
Why does it not make sense within the game?
Is it that you see no distinction between having levels in the Barbarian class and being a tribesman with a beard for example?
That a knight cannot be prone to great wrath in the throes of combat?
That honouring your ancestors, and them appearing to aid and protect you and your companions cannot be the behaviour of someone of noble birth?
Yes. The example which has been repeated is a character who is a Noble only and the character's class, Barbarian, has no effect on who the character is. It's just there because they needed a class.
On the contrary, their class informs and shapes a lot of the character. It provides not just pure mechanics like the proficiencies to represent that character's knightly training, but also both theme and mechanics. Like the character's penchant for going comprehensively nuts in a fight, and the presence of the ancestor spirits that the character honours.
Nobles don't adventure. Guild Artisans don't adventure. Hermits aren't adventurers. They need something after.
A character still has those skills they gained during their background. A noble still has a title and a guild artisan might still have their guild membership. But that isn't what they DO anymore.
The most important question to ask about your background is what changed? Why did you stop doing whatever your background describes and start adventuring?
So: "I was a knight, questing for glory for myself, gold for my family, and honour to my ancestors.
"Then I started adventuring."
You stop doing your background. You don't spend your days crafting. You don't spend your days alone in a hut. You don't spend your days managing your estate.
You might cease to perform some of the activities from your background, but you don't cease to become it. You don't stop being a Folk Hero when you swear your Oath of Devotion. You don't get kicked out of your guild when you start looking for more avenues of commerce in the wider world. You don't lose your taste for fine art or courtly manners the first time you go


on a bunch of gnolls.
You are primarily an adventurer now of a class and that class has meaning and identity.
You don't cease to become your background, with all its manners, bonds, flaws and habits however either. A character can combine both.
A Knight Barbarian might only lounge around the estate and attend parties in downtime (unless plot permits).
But even if you're trying to claim that they stop doing knightly things when they become an adventurer, are you also saying that they have to stop being and behaving like a knight?
Depends on whether you want to follow the rules as written.
OK. Now we're getting somewhere. You're insinuating that the suggested character concepts are
not following the rules as written?
That is something you can actually
prove that you're correct on.
Which rules are they not following? A quote would be ideal, but simply showing where in the PHB we should be looking would be fine.