Should they have to chase after Windows for the better part of its six month release cycle and MacOS for the better part of its twelve month release cycle just to make everything still work?
The fact that you run GIMP but the permission dialog doesn't appear until you go there in Terminal shows that something is, well gimped.
The fact that the default action for opening an unsigned program is deleting it or not running it with no explanation as to what to do to make it run shows that something is also gimped.
I think the stop before Catalina is where I get off the Mac OS update train.