You're holding others to a standard that you yourself haven't adhered to. Please go read the release notes. No mention anywhere that old undo data will be deleted.

https://neovim.io/news/2021/07/

Differences from Vim are also documented, but this issue isn't mentioned there either.

https://neovim.io/doc/user/vim_diff/

Also, what's up with people claiming that the undo files are stored under ~/.cache? That's completely made up. Or that persistent undo doesn't persist edit history, against what the docs say. Utter nonsense.

https://neovim.io/doc/user/undo/#_5.-undo-persistence

Users don't expect their editor to cause data loss after a routine `brew update`. Blaming users for that is unreasonable.

Nobody should have to read release notes to avoid data loss; there should never be a change such that someone who hasn't read the release notes (or missed a detail in reading them) to avoid such a problem.