Google has never limited itself to crawling and indexing just URLs that are in the sitemap. Such functionality does not exist, and I doubt that it ever will.
Sitemaps are fairly useless. They don't help with rankings. They rarely get Google to index pages it wouldn't otherwise index. Google really only uses them to choose preferred URLs, to specify alternate language URLs, and to give you extra data in search console. See The Sitemap Paradox.