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YouTube to restrict teenagers’ exposure to videos about weight and fitness
(www.theguardian.com)
This is a most excellent place for technology news and articles.
Weightlifting at certain young ages can be harmful. I'm not sure if that's part of the motivation as I'm not certain what the ages are, but that's something else to consider.
EDIT: Typo
Not more harmful than being obese. Or damage caused by body image issues.
https://ourworldindata.org/obesity
I didn't say that it is.
But to your second point, as others have mentioned, body image issues are likely the biggest reason for the change discussed in the OP.