It’s an outdated standard and a terrible idea nowadays, since it makes emails unreadable on mobile devices. I’d really love to use plain-text emails — hell, I use plain text for everything — but they’re sadly not a good idea today, unless you want your emails looking like crap on every mobile device.
The suggestion to use `format=flowed` doesn’t help, as the standard is ill-supported: https://fastmail.blog/2016/12/17/format-flowed/
I’ve researched the issue far and wide and the only way to have responsive, nicely wrapped emails is using HTML.