This is kind of a rehash of the argument on T5233, but...
- Admin blocks User:Malory indefinite when CookieSetOnAutoblock is on.
- User:Malory gets a BlockID cookie for his block id. Cookie expires after 30 days
- 10 years later, Malory has given his computer away to charity.
- New user goes to edit wikipedia. The cookie is long expired, but the localStorage entry is not. mediawiki.user.blockcookie detects this as user having "cleared" their cookies, and adds the cookie back based on LocalStorage.
- Person is now autoblocked because their computer was used by User:Malory 10 years ago.
To me personally this seems super excessive. I would suggest instead that the LocalStorage entry gets an expiry date explicitly added to it, so we don't use it if its super old.