But what is the overall risk, what is the risk to their families, what is the risk from lack of herd immunity? Vaccines are a shared risk.

The Delta variant is sufficiently contagious that there will be no significant herd immunity effect.

https://www.businessinsider.com/delta-variant-made-herd-immu...

This pre-print paper has some details on the risk of cardiac adverse events.

https://www.medrxiv.org/content/10.1101/2021.08.30.21262866v...

Even if we accept that risk is higher for others (which I am not), it does not follow that the vaccine should be mandated especially for a cohort where we know that not only is COVID not a meaningful risk for them, the vaccine itself puts them at higher risk. It is unethical to put these people at risk for the good of others. That is utilitarian and instrumentalizes human beings for the good of others. You can argue that they should have the option, but they should not be forced for someone else's good.

Teenage boys are also at the highest risk of anyone of being shot to death in trenches. That didn't stop us a century ago.

is this allowed to talk about here?

Yes, but usually this stuff gets flagged by users anyway.