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Gary Illyes has recently been pretty active in the StackOverflow forums and just posted in a Stack Overflow thread that Google, and other search engines, mostly ignore the lastmod tag found within XML Sitemap files.
He said:
The lastmod tag is optional in sitmaps and in most of the cases it's ignored by search engines, because webmasters are doing a horrible job keeping it accurate.
I believe Google's John Mueller was recommending to use the lastmod when going mobile friendly, to expedite the crawl and discovery of your mobile friendly pages. But John Mueller did downplay the use of priority and change frequency tags within the XML Sitemap file.
So probably using the lastmod date, even if you are honest about it, probably will be useless?
Forum discussion at Stack Overflow.
