We have some 301 and 404 permalinks in the sitemap, which is not good for SEO. Some (all?) of the 404s are posts that were reframed from Gawker, but the original post doesn’t exist anymore. This PR filters those out.

In general there’s no way to tell without looking for examples, and there can always be some corner cases hiding.

Looking through the code the obvious 404/301s are excluded now

Few things more frustrating for both visitors and search engines than to hit 404 error pages. It’s the dead end road of a lot of websites, and you don’t want your visitors to go down that road. If you have too many of these dead ends on your website, that’s a clear sign of bad maintenance.- Yoast

While it crawls your site, Google will encounter errors. It’ll usually just pick the next page from the pile when it does. If you have a lot of errors on your site during a crawl, Googlebot will slow down. It does that because it’s afraid that it’s causing the errors by crawling too fast. To prevent Googlebot from slowing down, you thus want to fix as much errors as you can. - Yoast