Google is good at its job. Sometimes, a little too good – indexing pages it was never meant to find. Undesirable URLs that are not contained in your sitemap. Orphaned URLs not linked on your website.

Google admitted in a video published on its own Search Central YouTube channel that if searchers do not use your page(s) in the search results, those pages may stop showing up in the search results.

With the Google Search Console manual submission issues with getting pages into the index, John Mueller from Google said on Twitter "If your site relies on manual index submission for normal content, ...

Google’s John Mueller answered a question about why Google indexes pages that are disallowed from crawling by robots.txt and why the it’s safe to ignore the related Search Console reports about those ...