Any documentation changes that update identifiers (content like headings, etc. that have an anchor) now require updating a redirects mapping doc/redirects.json or nixos/doc/manual/redirects.json for the nixpkgs and nixos manuals respectively. If not updated, CI should fail (given the last CI run was after this PR was merged).
This is part of an effort better described in this announcement.
To make this less inconvenient for contributors, a helper command is in the works which would remove the requirement to manually edit the aforementioned JSON mapping.