GDPR partly covers this since it's stated that the user must get information about how personal data is used in a clear and easy readable form. But I guess, there's some wiggle room how to interpret that. The law actually suggest that the industry could come up with symbols – like on food packaging. Your website could have a bunch of standardized icons in the footer to inform you how data is used, but since we don't have that it seems like the industry didn't like that idea of transparency.