@Jim I honestly have always liked this idea. Even for things like video games. Remember when Rock Star released the uncensored version of Manhunt on their own website? All companies would have to do is provide an optional patch for people that want to play the original experience without any ratings board involvement.
This is similar. Not everyone is mentally equipped to handle things beyond Disney level like an adult (and sometimes not even then). Blocking every individual that goes beyond that threshold is functionally impossible. If you let people have access to a one-and-done solution though, they'll take it, evidenced by Fediblock. Giving someone the option to go above a certain "rating limit" if they want, and making it clear that doing so will allow for posts containing x/y/z, gives more user choice and freedom.
As far as advertisers go, I always wondered why YouTube didn't actually try to get advertisers who were willing to put their ads on age restricted videos. Clearly, there are plenty of them out there judging by individual YouTuber sponsorships. They must already do something similar but in reverse for things marked as YouTube Kids... You wouldn't want to be advertising MW2 to a 6 year old.
The whole reason why I fucked off of Twitter and came here to begin with was to get the unrated version of the game, so-to-speak. I think it's a good move to do something like this. Make it easier for the user to customize their experience and give more freedom in the process. It's like needing to log in to a booru and check a box to see certain content, which has worked pretty well.