Earlier quoted context omitted.
Actually I didn't know; thank you for telling me! (That also makes me realize that I could likely do it in a much easier way than I currently am...) What are the email clients that allow that kind of behavior?
Well, essentially, all of the traditional text-only clients do (such as Mutt, which is what I use), I think Thunderbird does as well, beyond that, no clue, but there probably are others. Traditionally, if you chose to reply to an email, the mail client would put the mail you were responding to into your editor, with > marks in front of every line. The only socially acceptable way[1] to write your answer was to insert…
> Clueless developers and users around the turn of the millenium then somehow didn't understand the purpose of providing you with a quoted version of the email you replied to, and started putting the reply above the quoted mail, thus ending up sending a copy of the mail they just received back to the person they received it from ... and nobody seemed to notice how idiotic an idea that actually is, and so it became sortof the new norm in large parts of the internet to attach large blobs of completely useless and often close to unreadable text to every mail, up to the point where some people even invented justifications for the behaviour that mostly tend to describe how this "feature" allows them to work around the lack of proper thread handling in their mail clients.
Yes, I think the rationale must run something like "What if user A deletes the entire conversation they were having with user B, and user B replies to the deleted conversation, but user A doesn't know what the reply is about because they deleted the entire conversation, have a terrible memory, and are too embarrassed to ask user B to explain what's going on? Let's just send the entire conversation as quoted text every time."