Google has quietly removed the features that allowed you to restrict results to visited pages and pages you haven't visited yet. "Visited pages" and "not yet visited" are no longer included in the search tools menu.
Here's a screenshot from 2013: 9 search tools.
A screenshot from 2014: 4 search tools.
And here's the same menu today: only 2 search tools left, at least for now.
I'm sure that Google will soon remove "reading level" and "verbatim", since they're probably used by a small percentage of Google users, just like all the other advanced search features. Google now focuses on answering questions and natural language understanding, so these features for power users are no longer important.
Here's a list of cool features removed by Google in the past 5 years:
* finding sites with images
* finding search results in foreign languages ("translated foreign pages")
* restricting results to forums, reviews, blogs
* finding recipes (the feature is still available here)
* removing shopping sites from the list of search results
* blocking search results
* quick view for PDF files and Office files
* timeline view for search results.