There are several packages in portage which depend on net-print/cups, but all they really need is libcups.so.2. The ones I think about are binary packages, some for example ship libcef.so which is precompiled with cups dependency. They usually don't do any printing, so there's no harm done when the whole cups subsystem is not functioning - they only need a lib (maybe even stub would do) to run.
Two examples I noticed are media-sound/spotify and net-im/signal-desktop-bin (see https://bugs.gentoo.org/642008). It can possibly affect also things like www-client/google-chrome, but it has print features. There is a lot of things in portage which depend on cups, but probably don't use them, but I didn't check them.
Cups has a lot of dependencies, including filters, pdf stuff like poppler and so on. As a workaround, it is possible to emerge cups --nodeps (it builds fine), but portage is not happy with such tricks and it would mean adding a lot stuff to packages.provided. USE=minimal flag would be helpful in case of systems which don't need it all. Alternatively a separate ebuild providing only libcups.so could be added.
Steps to Reproduce:
1. emerge spotify
You get cups with all its dependencies.