> but AFAIK a simple forum where kids can talk to each other about familial abuse or whatever would also qualify.

I'm currently scrolling through this list https://en.wikipedia.org/wiki/Social_media_age_verification_... and it seems to me these are primarily focused on "social media" but missing from these short summaries is how social media is defined which is obviously an important detail.

Seems to me that an "easy" solution would be to implement some sort of size cap this way you could easily leave old school forums out.

It would no be a perfect solution, but it's probably better than including every site with user generated content.