I believe Google changed the sitemaps-related search developer documentation last week. I am not sure if any of the content specifically changed, but Google did rename some of the pages and create new pages, maybe to consolidate the content and make it easier to understand and access.
Google also added a new section named combining sitemap sections. Google also replaced the "split up your large sitemaps" with "manage sitemaps with sitemap index file." I did not dig through each file to see exactly what changed but you may want to.
Here is a side-by-side of the old structure versus the new structure, with the added "combining sitemap sections" in the sidebar:
Gary Illyes from Google confirmed on LinkedIn, "nothing changed, we just cleaned it up and reacted to some user feedback (like doc about combining sitemap extensions)."
Forum discussion at Twitter.
Update: Google told us "based on user feedback, we revamped our documentation about sitemaps. Notably, we reduced duplication between the sitemaps protocol and our documentation, added more examples to our documentation about sitemap extensions, and added a new document about how to combine sitemap extensions."
Update 2: A bit more from Google on this:
Just to be clear about the rationale behind that new doc: we got documentation feedback asking for an example of combining extensions. That's the only reason we added that new doc.
— Gary 鯨理／경리 Illyes (@methode) February 15, 2023
Long term I have no problems with working together to improve the sitemaps protocol, just not now
