Buttondown Documentation
Buttondown's default stance is to support all content so long as it is not harassing, damaging, or otherwise illegal.
However, paid newsletters are a bit of a different story. Buttondown uses Stripe to process payments; Stripe has some restrictions on adult content. Specifically, it prohibits:
Pornography and other obscene materials (including literature, imagery and other media) depicting nudity or explicitly sexual acts; sites offering any sexually-related services such as prostitution, escorts, pay-per view, adult live chat features; sexually oriented items (e.g., adult toys); adult video stores and sexually oriented massage parlors; gentleman's clubs, topless bars, and strip clubs; sexually oriented dating services
A couple notes on this: