They are also typically browser-specific extensions that were never cross-platform in the first place, features added based on proposals that were not in the end accepted (such as the Object.observe/unobserve API), or features from the Old Times™ before the specs were fully defined (and therefore typically also not cross-platform).
You've also got a bunch of deprecations for things that were in the spec, will almost certainly be supported forever, but are now seen as bad API design for one reason or another - usually because they don't handle edge cases correctly for historical reasons, or the name doesn't reflect what the function actually does. Unless any of these features actively leads to a security issue, they're very unlikely to be removed.
