This package is dead and stuck on unstable version so odd-unstable policy tried to downgrade it, even though it was intentionally pinned on unstable because no stable version is available.
continuation of #109595
pkgconfig was aliased in 2018, however, it remained in
all-packages.nix due to its wide usage. This cleans
up the remaining references to pkgs.pkgsconfig and
moves the entry to aliases.nix.
python3Packages.pkgconfig remained unchanged because
it's the canonical name of the upstream package
on pypi.
Jan Tojnar