Earlier quoted context omitted.
> 80-char lines are too short for desktop displays and too long for small phones (leading to awful linebreaks), which makes me the guy whose emails never look right. RFC 3676 [1] describes the format MIME parameter for text/plain content type documents which solves this issue. Unfortunately, it does not work well when sending inline patches via email since it introduces trailing whitespace. [1] https://www.ietf.org/r…
format=flowed has many other problems, aside from mangling mails. It gets lost easily; intermixing mail clients that handle it with those that don't (or have been intentionally configured to ignore it because it mangles mails) produces poor results; it often breaks signatures; it makes it harder to quote and reply to emails; in general, it means the person on the other end will not see exactly what you sent.
How, unless the MUA is modifying quoted text beyond prefixing each line with >.
> intermixing mail clients that handle it with those that don't (or have been intentionally configured to ignore it because it mangles mails) produces poor results
I participated in many lengthy discussions in deeply nested threads on usenet with an email client (Thunderbird) that had format=flowed enabled for the text/plain posts that I was sending. I never noticed any issues when viewing posts I made or other posts that were descendants of my posts. That is, no problems with line wrap or other rendering issues (either when looking at it in Thunderbird's view pane or looking at the raw message text).
> it often breaks signatures;
It shouldn't break them if the client applies the signature itself.
> it makes it harder to quote and reply to emails
I don't see how this would be the case. When you reply to an email, the MUA should prefix each line in the body of the original message with a > character. Whether those lines have a single trailing whitespace character or not shouldn't make a difference in terms of quoting the original text.
> it means the person on the other end will not see exactly what you sent.
Unless you have text that requires wrapping at specific line lengths, they will see what you sent. If their client supports the flowed format, the lines will be rewrapped to your screen width. If their client does not, then they see the original raw text with trailing whitespace at the end of each line (other than the last line of each paragraph) which essentially is not visible to the reader unless they've configured their client to display a marker for trailing whitespace.