I just looked at the binary packages on ftp.netbsd.org (from a shell), and looked at the bulk build report.
Some of the packages have modification times that are before the branch, going back to the beginning of 2018. That should have been ok, because we try not to have ABI changes on the branch, but really that rule starts at the first release. There are gcc packages that appear to predate the branch. To pick a random example, the gnome-online-accounts-3.28.0nb7.tgz package was built on January 13, but needs libstdc++.so.7 via the gcc6 libs package. However, it doesn't seem to explicitly depend on gcc6-libs. So my suggestion would be to rm all the packages that have dates before 'digest' on the pbulk machine and kick off a new build. That will end up rebuilding a lot of packages, but with any luck not all of them. Manuel? This may or may not fix rust, but rust 1.31.1 built for me on January 9 on a netbsd-8 (not 8.0) machine.