Note, however, that the plain text form has for a long time typically I believe been the output of a machine conversion from an XML vocabulary, and now always is. I’m not deep enough in the weeds to know all the whens and the distribution of formats, but RFC 2629 (June 1999) defined the first XML vocabulary, and RFC 7990 (December 2016) completed the process of declaring an XML format the canonical source, rather than plain text. To learn more, start at https://www.rfc-editor.org/rse/format-faq/.
Consequently, new RFCs are now published most obviously as full regular HTML, like https://www.rfc-editor.org/rfc/rfc9110. The plain text form is still available (https://www.rfc-editor.org/rfc/rfc9110.txt), but it’s no longer paginated and may be incomplete, as specs may include SVG graphics. (RFC 9110 uses ASCII art in some places, e.g. sections 3.6 and 3.7. I am no connoisseur of recent RFCs, but don’t know of any using SVG yet to give an example. I don’t know what the text form gets in this case.)
Most germane to this discussion now: older RFCs got converted to lightly-marked-up-but-mostly-plain-text HTML via rfc2html, and exceptions and tweaks and whatnot had to be made regularly, because in the past the RFC document format was not designed to be machine-readable, and it showed in irregularities that are not acceptable in a markup language.
So in summary: yes, you can get some inspiration from RFC documents, but don’t mistake that textual format for an acceptable markup language, because it wouldn’t actually work.
Consequently, new RFCs are now published most obviously as full regular HTML, like https://www.rfc-editor.org/rfc/rfc9110. The plain text form is still available (https://www.rfc-editor.org/rfc/rfc9110.txt), but it’s no longer paginated and may be incomplete, as specs may include SVG graphics. (RFC 9110 uses ASCII art in some places, e.g. sections 3.6 and 3.7. I am no connoisseur of recent RFCs, but don’t know of any using SVG yet to give an example. I don’t know what the text form gets in this case.)
Most germane to this discussion now: older RFCs got converted to lightly-marked-up-but-mostly-plain-text HTML via rfc2html, and exceptions and tweaks and whatnot had to be made regularly, because in the past the RFC document format was not designed to be machine-readable, and it showed in irregularities that are not acceptable in a markup language.
So in summary: yes, you can get some inspiration from RFC documents, but don’t mistake that textual format for an acceptable markup language, because it wouldn’t actually work.