Google Is Indexing much fewer pages these days - how well are you indexed?
In the past, Google used to show how many pages were in their index - in fact, it was was a point of pride that they had the biggest index. A large number of pages were in the "supplemental index", which Google reps said was eliminated at a certain point.
One thing you'll notice if you look at Googles homepage today is that they no longer feature pages indexed. Also the Panda updates were a big sign of the shift in direction in what Google shows. Lastly, Google used to show "not selected" in Google Search Console, a feature that they took away.
Especially if you manage larger site(s), in terms of number of pages, it is worth doing an indexation check if you have not done so in a while. Although a complete indexation check takes some time and a variety of techniques, here are some quick tips.
Use Google Search Console: A good starting point is to check the crawl report in Google Search Console. To access the report, log in to GSC, select the site you want learn about and select:
"Crawl" > "Crawl Stats". Check the "average" of your "pages crawled per day" - is it what you expect for a site of your size.
Check Google: Run "site:" commands and page titles for some of your most SEO significant directories and use the pagination to see what shows up. Are your SEO significant pages showing up?