According to upstream (http://mumps.enseeiht.fr/doc/userguide_5.1.2.pdf, section 2.5.2 on page 9) for current mumps the latest version of scotch >=scotch-6.0.1 is recommended.
I closed the PR some time ago.
Due to upstream having updated to 5.2.1, the userguide is now at http://mumps.enseeiht.fr/doc/userguide_5.2.1.pdf.
Information about dependency on scotch is now in section 2.6.2 on page 10.
According to that, mumps (5.2.1) is compatible with scotch 5.1.x and 6.0.x, but not with 6.0.0, due to some problem with this version. They recommend, however, to use scotch >= 6.0.1.
This is blocking <sci-libs/scotch-6 cleanup.