Comment by kelnos

AFAICT neither has any default for `undodir`; the user must set it explicitly. I think a user could be forgiven for assuming that it was fine to use the same value when trying out a migration from vim to neovim, and that neovim wouldn't actively delete vim's state data.

Sure, maybe they should have backed it up or used a different directory, but I think it's reasonable to expect that many people wouldn't. If a software developer is only going to consider the scenario where their users are thinking/behaving exactly as they are, then they aren't particularly good at what they're doing.