block-all-mixed-content is a CSP directive that causes Chrome to hard block all http resource loads on https sites. After the launch of autoupgrades for passive mixed content, the directive is a no-op since passive (image, video, and audio) mixed content is autoupgraded to https before block-all-mixed-content is evaluated (and fails to load if not available over https), and active mixed content is hard blocked by default. block-all-mixed content still has an effect when a user has allowlisted a site (using the "Insecure Content" site setting toggle) to allow mixed content, but that is a fairly niche use case (and it seems unlikely that sites are relying on that functionality). block-all-mixed-content was previously defined in the MIX spec, but was marked as obsolete when MIX and MIX2 were merged and the concept of autoupgrades was introduced. It is already marked as deprecated in MDN docs.
block-all-mixed content is already marked as obsolete in the Mixed Content spec, is a no-op in most cases, and removing it would simplify Chrome's mixed content handling code.