> Online platforms should do what nearly every other publisher does and provide a rating for their content.

That only happens to "publications" of particular forms where state regulation has mandated it, or enough noise was made about state regulation mandating it (or simply censoring content) was made that the industry adopted a rating system as a way to discourage that (and in the latter case, there are always plenty of publishers that don't make use of the industry rating system, either at all or at least for selected publications in the field to which the ratings nominally apply.)

> They provide a "kids" profile populated with their own curated content if that's the kind of thing I want and for everything else they provide ratings

Netflix does not provide ratings for "everything else". Most of what they carry has either MPAA or TV Parental Guidelines ratings, and if it has such ratings they provide them. But they have content which does not have such ratings, which is simply noted as not being rated. (Of course, if "not rated" as an option is a valid to comply with your "you must have ratings in an HTTP header" law HTTP header, then it is trivial to comply and provide the "not rated" header for every piece of content, but this doesn't actually achieve anything.)