sometimes we remove packages but the dependencies are left, though no other package needs them. Now, if we want to clean the distro, how do you do that? How people do it in slackware?
In debian, for instance, I can track them with the tool: deborphan...

Well, slack doesn't have any automatic dependency checking. Which means that you can't automatically track down any redundant library packages. On the other hand, you won't ever have had anything installed automatically because something else depended on it, so there won't be so much to get rid of anyway.

Generally all the shared libraries that other packages need will be installed in the initial slack installation. If you really want to prune out those that you aren't using then go ahead, but as far as I know there is no automatic tool for doing it.