I'm a forever DM so I've got so many unused character ideas that turn into either NPCs or if a player is like "I forgot to make a backstory"

But with the amount of improv I do as a DM it's very little effort for me.

Though I agree it should be discussed as part of session 0. As it'd be rude not to.

Session 0 for my groups is always the same really, we sit around boucing ideas off of each other for characters, things we don't want to be touched on during the campaign, and what kind of campaign we want.