Fun. Apparently there was a leftover file,
-rw-r--r-- 1 root root 411 Oct 20 2019 /etc/systemd/network/99-default.link
...that didn't have a valid configuration syntax for the current version of systemd-udevd:
systemd-udevd: /etc/systemd/network/99-default.link: No valid settings found in the [Match] section, ignoring file.
Removing that file solved the problem (though right now I'm not sure of the defaults from /usr/lib/systemd/network/99-default.link are now used nor not.)
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