Via Rick Thomas on the Debian Users mailing list:
In any case, the solution I came up with is
apt-get --purge install -y sysvinit-core dbus- glib-networking- libgtk-3-0- apt-get --purge autoremove
Note the trailing minus-signs on dbus- glib-networking- libgtk-3-0- These packages need to be deleted in the same pass as sysvinit-core is added.
I haven't had the chance to try this yet, but it certainly looks reasonable.
Later: here's what you should actually use:
apt install sysvinit-core dbus- elogind
Looks like XFCE4 is not going to work without systemd, though.
From the Debian init-diversity mailing list:
"I just wanted to speed up the backport as much as possible -- as-is, Buster is useless for GUI users."
So, we'll need to wait for backports.
apt install sysvinit-core elogind/unstable libelogind0/unstable libelogind0:i386/unstable libpam-elogind/unstable libpam-elogind-compat/experimental
might work, but is unlikely to make you happy long term. Wait for backports.