Outcome from 2018 SEO project with Go Fish Digital:
Most of our sites do not have XML sitemaps. Sitemaps are important so that crawlers know how to crawl our site and understand how pages relate to each other. A sitemap also guarantees that pages are not missed by the crawler, since it'll look at everything that's in the sitemap. A lack of a sitemap is normally a gigantic problem, but Wikimedia sites are so popular that we've gotten away not having one.
Many of our sites are gigantic, and XML sitemaps can only contain a maximum of 50,000 items. To compensate for this, a kind of "sitemap of sitemaps" can be created, which would be more than sufficient to contain everything. A sitemap like this should be created.