Consider converting the original text (maintaining the author’s original line wrapping and indentation) to base64. Has anything been “inserted” into the text? I would suggest not. It has been encoded.
Now consider an encoding that leaves most of the text readable, translates some things based on a line length limit, and some other things based on transport limitations (e.g. passing through 7-bit systems.) As long as one follows the correct decoding rules, the original will remain intact - nothing “inserted.” The problem is someone just knowledgeable enough to be aware that email is human readable but not aware of the proper decoding has attempted to “clean up” the email for sharing.
Infinite line length = infinite buffer. Even worse, QP is 7-bit (because SMTP started out ASCII only), so characters >127 get encoded as three bytes (equal, then two hex digits), so a 500-character non-ASCII UTF8 line is 1500 bytes.
It all made sense at the time. Not so much these days when 7-bit pipes only exist because they always have.