Thank you very much for your attention.

I am afraid that by installing either logind or libpam-elogind, policykit-1 is still being eliminated (and consequently synaptic because of its dependence).

# apt-cache madison policykit-1
policykit-1 | 0.105-25+devuan8 | http://deb.devuan.org/merged beowulf/main i386 Packages
policykit-1 | 0.105-25+devuan8 | http://deb.devuan.org/merged beowulf/main Sources
# apt-get install --reinstall policykit-1
upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 1 not upgraded.
# apt-get install logind
The following additional packages will be installed:
elogind
Recommended packages:
policykit-1
The following packages will be REMOVED:
colord libpam-ck-connector libpolkit-backend-consolekit-1-0 policykit-1 policykit-1-gnome synaptic
The following NEW packages will be installed:
elogind libpam-elogind
0 upgraded, 2 newly installed, 9 to remove and 1 not upgraded.
Need to get 0 B/825 kB of archives.
After this operation, 7,106 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
# apt-get install libpam-elogind
The following additional packages will be installed:
elogind
Recommended packages:
policykit-1
The following packages will be REMOVED:
colord libpam-ck-connector libpolkit-backend-consolekit-1-0 policykit-1 policykit-1-gnome synaptic
The following NEW packages will be installed:
elogind libpam-elogind
0 upgraded, 2 newly installed, 9 to remove and 1 not upgraded.
Need to get 0 B/825 kB of archives.
After this operation, 7,106 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.