Hostname field isn't a hostname but a base URL

Yes, this happens. Perhaps, a field label should be changed?

I've updated the label text.

trackleft2 → changed the visibility of the branch 4.x to hidden.

I've added a post_update implementation to update any configuration that might have a url value with a trailing slash.
I've added a form validation to set an error if the form value for url contains a trailing slash.