Nobody is trying to age restrict the safari app (like a movie theater), but I don't see why websites or parts of websites can't be restricted like individual movies are as they are given a rating by an authority.

But other apps entirely like dating apps? or only fans? can probably be entirely restricted to some age.

This isn't even a hard question. The movie theater is open but movies that are rated R are not. In this case, Reddit.com is a movie theater, subreddits are movies. The website might be open, but not every subreddit is. This is in fact how Reddit already operates, age verification is just a joke right now.

What is the fight over? It semes like some people are fighting over age restriction at all, and others are fighting over the specific exchange of age proof mechanisms.

???