A sitemap earns its keep on large sites, on sites with weak internal linking, and on newly published content that needs finding quickly. It should contain only canonical, indexable, 200-responding URLs: including redirects, noindexed pages or 404s teaches the engine that the file is unreliable and reduces how much attention it gets.
The `lastmod` date is only useful if it is honest. Bumping every date on every deployment is a common accident that destroys the signal's value, and crawlers learn to ignore it. Split very large sitemaps into an index of files by section, which also makes the Search Console coverage report readable by content type.