The page_restrictions table contains a pr_user field:
-- Field for future support of per-user restriction. pr_user int unsigned NULL,
This field was introduced in 2007 by changeset https://phabricator.wikimedia.org/rSVN19047 . I found no relevant tracking ticket to actually implement this per-user restrictions feature. As such, probably the field should be removed from schema as it was never used and probably will not be.
Alternatively, if the field is still in use somehow (does not seem so), such usage should be documented in the description.