From lintian:

W: rshim-user-space source: build-depends-on-obsolete-package Build-Depends: pkg-config => pkgconf
N:
N:   The package build-depends on a package that has been superseded. If the
N:   superseded package is part of an ORed group, it should not be the first
N:   package in the group.
N:
N:   Visibility: warning
N:   Show-Always: no
N:   Check: fields/package-relations