https://github.com/neovim/neovim/blob/fc3f0041fbe01c96f38224...
The feature that was broken is called persistent undo. The feature was inherited from Vim. In case it's not obvious from the name, nowhere does it state in the docs for persistent undo that data may be deleted at any time.
https://neovim.io/doc/user/undo/#persistent-undo
That Neovim changed the default storage path for undo files is completely irrelevant to whether the contract for persistence should be broken.
I would consider deleting my persistent undo state, which would be something that I would have specifically enabled, to be "severely impacting".
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.