To further what Admin has said,
If you have a page which is no longer linked, it will remove the URL, an easy fix is to add a link to your html sitemap, and the crawler will read the sitemap page, so even if you remove the link to that page, the crawler will not remove the URL as it finds the URL in the sitemap. If you delete pages, and remove the links, the crawler will see the broken links in the sitemap.html and thus remove them.