It’s the sort of feature that is fairly useless unless it gains very widespread adoption, and also often quite nebulous about how it should be used. My feeling

It’s the sort of feature that is fairly useless unless it gains very widespread adoption, and also often quite nebulous about how it should be used. My feeling is also that the sorts of people most likely to be willing to implement this kind of thing unprompted are likely to be producing content that can benefit from such stuff less than average. Also, to repeat, there have been specs for this kind of thing in the past. They haven’t gone anywhere despite significant interest and investment. Granted, some were probably hampered by being unnecessarily complicated, but basically no one used them. (Examples: PICS, in in a couple of slightly different veins but conceptually similar, Schema.org’s contentRating property, and P3P.)