Gary Illyes from Google confirmed that XML Sitemaps are still the second most important source Google uses to crawl and discover URLs. He confirmed this on Twitter the other day after someone quoted him as saying this back in 2014.

Gary said this at an event just recently:

Then someone said he also said this back in 2014, which he confirmed is still the case today:

So if crawling isn’t something you want to depend on, make sure to get those XML Sitemaps going.

Forum discussion at Twitter.