In https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=2137a49dd6742a55a08ca9abe2d545de11a4de33 I wrote by accident "Closes: 638732" instead of the Bugzilla URL and got "remote: WARNING: invalid/unsupported bug ref: 638732" But the server accepted it. It would be more useful, to reject commits with this pattern, when starting on a newline.
I occasionally reference external bug trackers using "Bug:". Rejecting commits based on that would be annoying, unless someone can suggest a better way to make such references.