When I ran apt-get purge systemd I noticed it removed some mate components and lightdm, easily reinstalled before I had an issue.
When instructed to edit sources.list - suggest to remind to check apt.conf for a Default-Release setting, it's a slight problem later if not.

I guess I skimmed over the instructions for apt-get purge libsystemd0 - I was running xorg.  Purging this removes most of what makes the system functional.  Found https://devuan.org/os/issues suggesting not to worry about it.

In the end I have a fully stable (as it seems) devaun ascii system with mate-desktop migrated from debian stretch!!   It does work - thanks greatly.