Manual:$wgCookieSetOnAutoblock

Details
The cookie that is set will expire when the block expires or after , whichever is shorter (the latter defaults to 30 days).

This means that cookies for infinite blocks will also expire at this shorter time.

The cookie is named 'BlockID', prefixed with .

It's value is just the block's ID and no other data.

For users with LocalStorage support, a matching entry is created in LocalStorage.

In case the cookie is deleted, it will be recreated from this (at the time when the user opens the page-editing form).