This reads to me like a case of blame on both sides. Putting data you don't want to lose in ~/.cache is PEBKAC, but if that fact is incidental and NeoVim would have removed the undo history regardless of its filesystem path, then the point remains valid that NeoVim is deleting user data that isn't its place to delete.

A lot of the comments here are getting caught up in legal arguments which may or may not be valid. Those are irrelevant. No one is taking the NeoVim developers to court; the post is merely warning that they knowingly accepted behavior which can cause harm. The question is whether or not that choice was responsible, not whether it's legally actionable.