Best way to deal with node data that should only be displayed in views
A couple strategies here...

One is the obvious one you mention: Don't publish the 'Slide' content type, so it never appears on the 'live' site.

Another is that you don't make special 'slideshow' content types. You just add fields to your normal content types that will be displayed in a slideshow display mode. Then you hide them for other display uses like Default and Teaser.

Drupal's flexibility with this type of scenario is one of its major strengths. But it also means that site editors have to remember which field does what, or which type of nodes not to publish.