Seriously this opens up so much creativity for the DM.

"This person seems familiar to you, but you're not sure why."

"You think you've been here before, but you're not sure when."

"Being on a ship feels like home, but you have no memories of ever being on a ship."

"Upon seeing the ruins of the village you find yourself overcome with sorrow, you feel like you've lost something important but you don't know what."

Part of the adventure can be a quest to reunite them with their lost memories.

In other words, "My backstory is whatever you want it to be".

If you were the DM and this bothered you, the player just gave you powerful ammunition.

You could even have it so whenever the player entered a shop in his home town, the shopkeepers looked at him with disgust and refused to serve him. The DM wouldn't even have to necessarily come up with a reason. Just, that the player is extremely well known among the locals and they universally think he's absolutely disgusting and want nothing to do with him.

100% ok. Be prepared for weird stuff happening to you whenever I need a plot hook.

As long the player is open to whatever the DM decides their characters history is, or they have collaborated on an outline or even a detailed history the other players just don't know about, that is all fine and can be really fun for everyone.

If they are doing it with no heads up and are not going to play along with whatever the DM decides happened in their past, then no. But when it comes to DnD, I normally let stuff play out and only stop play if something is clearly going poorly, or might make other players more uncomfortable than they are willing to be. I only play with friends, so things rarely end up being anything close to the worst they theoretically could have gone, socially.

Acceptable but only if vague memories and impressions occasionally surface and lead to some plotline where you eventually rediscover yourself

Though that would work much better if you shared with the DM I suppose

Gonna be honest, I usually make my backstory like 2 or 3 sessions in, that way I get a feel for the character and can do something that makes sense. I usually start off with a rough idea, but I like incorporating themes from the story into my background

... barely an inconvenience.

I was literally born yesterday.

I never create a backstory. All my characters had personalities, different ways of solving problems, and I acted out the characters in their own unique ways.

All my characters are built around an idea. Ideas like "Kobold snake oil salesman", "Necromancer Child Edutainer", "Skaven Engineer", "Communist Dwarf", "A delusional ghoul named Jeff Bezos", etc.

Since, I do a lot of improvisation, not having a back story allows me to adapt my character to the story.

Why is the whole village chasing us with pitchforks again warlock? What the fuck did you do?

Imagine finding out Orks killed your parents at level 10, long after that favored enemy bonus will be useful.

This falls squarely under “play to find out,” so it sounds pretty great.

If that's the card they want to play then it's only going to hurt them until they start giving me something i can run with.

As a general rule, im willing to match and even exceed the effort players put into their PCs, provided they have done anything more than a statblock with a name.

But if they just have the statblock and name then I'm not going out of my way to customize content and quests specific for them.

https://2e.aonprd.com/Backgrounds.aspx?ID=182

I got bashed in the head and woke up here with no memories.

If I were DM, congrats! You're hidden backstory is that you were one of the most depraved rodeo clowns to ever have existed. You were patient zero of a necromantic crotch-rot epidemic plaguing your birth-nation to this very day. You traded your memories for the cure. No one recognizes you because you're not wearing your signature make-up and are wearing more than just chaps and a crazed grin.