The wiki contains an [NSFW] label in front of links to websites with inappropriate material. That is your content warning.
Though, I will admit, I've found myself to be a bit lax in remembering to apply those warnings sometimes, and I no longer peruse the sites of all the artists I see because it takes so much time.
But the basic thing to know if that Pixiv is an NSFW website, and all artists may have explicit content on their site. If you're unfamiliar with a link, you shouldn't click it in an inappropriate place. And if you're too young to see that sort of stuff, you should get someone to check it out for you or something. We do our best, but this place is run by people. The content warning labels would be placed on things by people. So a more "real" system wouldn't really do anything.
However, if you see an artist's wiki entry that is missing a label, feel free to add it. In fact, we encourage it. That's why the wiki is editable by all users.
(And yes, I am assuming that the user is linking to the artist's site via the wiki. Anywhere else they could get the link is not under our jurisdiction.)