I walk up to the bartender and ask him about rumors.

Bartender: "You son of a bitch, I told you if you ever come back, you better be able to pay your tab!"

DM: "Alright. Everyone else, please make up and write down two things about this player's backstory and let me know. Do not tell each other what you wrote. I'll choose which ones I'll use plus may throw in a couple myself" evil laugh

Yuuuup.

Fiend warlock player did not like learning how I used his minor backstory. (He actually loved it.)

He knew he needed to save a tree. He didn't know he was the one to burn it, nor that he did so because it was a source of power to his mother, who was a night hag.

So much fun.

He saved the tree and even managed to change his Patron, but is too scared to confront his mother again.

Look, if I wanted to write 100% of your backstory for you, I'd have been a player. Ttrpgs are meant to be collaborative. I already have a shit ton to do as GM, you could at least throw me a bone. Even if you want to do an amnesia plot, that doesn't mean you the player doesn't know anything at all.

I once had a player in my game play a changeling who swapped places with someone, then forgot they were a changeling. So naturally, I had the rest of the party meet the original without her. That was a fun reveal.

I'd start scribbling notes on my character sheet arms like in Memento.

I actually agree, that could be pretty fun. That said this puts a lot of work / pressure on the DM so it is definitely something to be discussed rather than used to be lazy. (To be clear, I am not reading this into what you said. Just saying it out loud is all)

The idea of planting a potential scorched home only to find out it was the location of your favorite chalupa place later on

This is pretty much what's happened with my latest character.

I'm playing a reborn/dragonborn, who had previously tried to fight Strahd but lost. And a part of that loss means that my character lost a good chunk of his memories of fighting Strahd before.

But Strahd still remembers my character. So it has lead to some interesting interactions.

Or you can mess with the player. Someone turns up chasing them for child support.

Oh so many cool prompts. It reminds me of thousand year old vampire. Exploring those really do sound like fun!

GURPS has a specific disadvantage that is essentially this.

Baldur's Gate intensifies

See: Disco Elysium.

You forgot how to piss, shit, bathe, heal, feed and dress yourself when you gave up your memories. In fact, you forgot how to walk.