APT: The method 'ssh' is unsupported and disabled by default.

Recent Debian (and Devuan) releases have disabled ssh and rsh protocols as possible transfer method of packages. The solution to permit them again is to re-enable them in apt configuration file.

Create your own config file /etc/apt/apt.conf.d/30-ssh-transport and paste the following:

Dir::Bin::Methods::rsh "rsh";
Dir::Bin::Methods::ssh "ssh";

Package transport over both protocols should be re-enabled now.