Bug#1072474: backintime-common: "sshfs" should be "dep" instead of "sug"

[c . buhtz]

It gives an error message dialog: "sshfs not found. Please install it 
(e.g. via "apt install sshfs")"

I don't like that behavior. The GUI suggest that this functionality does 
exist, the user invest time in configuring it and near the end that 
error comes up. From a view point of usability it is not friendly to 
users.
But you are the maintainer. It is up to you of course. ;)