For context, facebook is so dystopian when I login once every few years that I’m not sure I’ll ever use it again. And, I hate wading through the YouTube cesspool to find some educational content I like. But, I don’t think it makes sense to ban a/b testing or optimization in general. Some company could use it, for example, to figure out how to teach math to kids in a way that’s as engaging as possible. This would be “more addictive” technically.

That's a good point, I'm not 100% sure it's worth throwing away the potentially beneficial uses. There might not be a solution that's both feasible to implement and avoids banning useful things. In the end I usually come back to it being the parent's responsibility to monitor usage, limit screen time, etc., but it hasn't been working so well in practice.