Errors and SEO

5xx Errors: these are server-level errors. They are considered severe.

Don’t allow search engines to crawl the page
Harm UX, which in turn harms SEO
Harm the site’s health
Is a lost visit, as the page is unable to be tracked by Analytics software
Excessive or prolonged 5xx errors will harm rankings

4xx Errors: these are client-side errors. They are considered less severe.

Search engines attempt to crawl the page, depleting crawl budget
Harm UX, which in turn harms SEO
Harm the site’s health
Increase SEO bounce rate
Anytime a URL is renamed without a redirect, a 4xx error can occur
Page will eventually drop from Google’s index

301 Redirects: A URL was requested, but the server directed the request to another URL.

Affect the crawl budget if redirect chains are present (URL -> URL2 -> URL3 -> etc), or if redirects lead to a 4xx or 5xx level page
Are necessary to preserve page authority from external backlinks
Internal links should always be updated to avoid redirects