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:
Sitemaps are the second Discovery option most relevant for Googlebot @methode #SOB2019
— Enrique Hidalgo (@EnriqueStinson) June 15, 2019
Then someone said he also said this back in 2014, which he confirmed is still the case today:
Still true
— Gary "鯨理" Illyes (@methode) August 26, 2019
So if crawling isn't something you want to depend on, make sure to get those XML Sitemaps going.
Forum discussion at Twitter.