> * The support for SysV and LSB init scripts has been removed from the systemd daemon itself. Instead, it is now implemented as a generator that creates native systemd units from these scripts when needed. This enables us to remove a substantial amount of legacy code from PID 1, following the fact that many distributions only ship a very small number of LSB/SysV init scripts nowadays.
At the earliest possible opportunity, systemd removed everything they could that supported alternative init systems. So no, it is not trivial to take a Debian release and make it run SysVinit or OpenRC. In fact, it was so much work that the Devuan devs expended the effort to fork, including setting up a website, mailing lists, IRC channels, a bug-tracker, and all the other infrastructure necessary to maintain a serious, well-supported distro.
And as far as "upstreaming" to Debian, how is Debian upstream for SysVinit or OpenRC? Both of those are separate projects not owned by Debian.