First of all, RFC is just an abbreviation for "Request for Comment" and does not inherently refer to the IETF.
Secondly, the site you have linked is just displaying the content with the CSS-attribute
font-family: monospace. Your browser interprets this and as it is one of the nine generic font familys (
fangsong) it will ask the operating system for a fitting font. So it really comes down to how and on which machine you are viewing this.
It could be any of these or a different one altogether:
- Fira Mono
- DejaVu Sans Mono
- Liberation Mono
- Lucida Console