Debian has not in fact been systemd only; the GR that is being discussed in this thread replaces a decision by the Technical Committee from 2014:
https://lists.debian.org/debian-devel-announce/2014/08/msg00...
For the record, the TC expects maintainers to continue to support
the multiple available init systems in Debian. That includes
merging reasonable contributions, and not reverting existing
support without a compelling reason.
However, in practice Debian users tend not to replace systemd's PID1 with the sysvinit-core package, as it is installed on a negligible fraction of Debian systems.https://qa.debian.org/popcon-graph.php?packages=systemd%2C+s...