It looks like CT will (if you give permission) track your ride with a view to improving routing. You can see that in settings (in the app).

However, I'd imagine such learning is littered with potential problems. Have I turned off because the route is bad or have I had an invitation to visit someone?

Perhaps you could clarify: "Can't be ridden" can mean different things to different people. A simple example: heavy rain can make a beautiful, sunny-day ride very uncomfortable. 

CT uses data from OpenStreetMap - a mapping system maintained by volunteers so you can investigate the labelling of routes and change them. Not an easy task for some of us. :-)

If you are regularly hitting impossible roads then check that you have selected "Paved only".

In fact, I'd suggest using the website for planning, the app for navigation. 

One thing you can do is to record your route using something like RWGPS, then at home edit it in CT to build up a library of suitable routes.

It would be helpful if you could supply an example of a route or at least the part of the world you are in!