The outcome of this thread was that session management is possible
without policykit, at the expense of few little hacks.
Unfortunately, without policykit, the users are not allowed to
mount removable media like usb memory sticks. Few years ago the
permissions were handled in udev rules, but nowadays udev rules have
shrinked to one rule in 70-persistent-net.rules. Seems that everything
else is now done by default in udev and permissions are delegated to
policykit. It might still be possible to manage permissions in udev, but
I didn't try and re-installed policykit1, at least temporarily.
Devuan's policykit1 doesn't depend on systemd, sure, but it remains
a relatively obscure thing with its own configuration meta-language
AFAIR, and it goes in the way in places where simple file permission
would do a perfect job. It is clear, however, that some mechanism is
needed to allow normal users to mount removable disk partitions and the
traditional file permissions paradigm falls short in this case. Is there
any other case?
I guess when one double-clicks the removable media's icon, Xfec4
invokes /usr/bin/pkexec to get the permission. pkexec seems to be the
CLI interface to Policykit. Therefore it might be possible to substitute
Policykit with a hand-crafted script named /usr/bin/pkexec, invoking
sudo, for example.
Didier