RFC 1896 specifies text/enriched: https://datatracker.ietf.org/doc/html/rfc1896
And of course nowadays there is Markdown, which basically codifies a lot of earlier plain-text formatting conventions.
> Please don't say "there shouldn't be formatted text in email." That's not a reasonable position.
I don’t know about that, really. We got by fine with plain text messages for decades, and the formatting conventions were good enough to convey meaning and nuance.