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