Most SEOs that have been doing the search engine optimization thing for any amount of time know that Google can and likely will drop out URLs from the Google index. John Mueller from Google confirmed that this happens with Google Search on Twitter the other day.
John wrote "It doesn't really matter what happened, but yes, any URL can drop out of the index over time."
Even URLs that you deem important or one of the most prominent sections of your site can happen to see that URL drop out of Google Search.
The trick is to make sure to have the URL prominently linked to throughout the site's navigation and from other popular pages on your site. Google still may decide not to index the page later on, but you need to keep that URL relevant, unique and helpful for Google to still want to index and also rank the page.
Here is John's tweet in context:
It doesn't really matter what happened, but yes, any URL can drop out of the index over time.
— 🥔 johnmu (personal) updated for 2022 🥔 (@JohnMu) August 5, 2022
Forum discussion at Twitter.