Sitemap Checker

The most common cause is a security plugin or host firewall blocking bot-style requests to it, which reads as "missing" to some checkers even though the file is fine. The second most common cause on WordPress is a stale rewrite table: enabling a sitemap in Rank Math or Yoast, then never visiting Settings → Permalinks to save and flush the rules, leaves the URL 404ing even though the sitemap generator is switched on.

It is not strictly required if you have already submitted the sitemap in Search Console, but it is good practice. It is also how several AI crawlers that have no equivalent of a webmaster console discover it, since robots.txt is the one place every crawler checks first.

The <lastmod> date is how a crawler decides which of your pages might be worth re-fetching without downloading everything again. Pages updated recently and reflected in an accurate lastmod earn measurably more attention from both classic search re-crawls and AI answer engines than pages a sitemap has not touched in months.

A single sitemap file is capped at 50,000 URLs and 50MB uncompressed by the sitemap protocol. A site with more pages than that publishes a sitemap index instead, which is a file that just lists the URLs of several smaller sitemap files. Both are valid; a sitemap index just means the site is large enough to need splitting.