It turns out the root cause of this is systemd pulling all support for sysv back in March, and having systems silently break if they're still using it. So it's not just Ubuntu; every distro running systemd that still allowed init.d is likely to have this issue when to go to this version of systemd.
https://linuxjust4u.com/linux/systemd-260-sysv-removed-linux-upgrade-guide/