Note the irony that you cited a document whose canonical version is a plain text file written in a typewriter-compatible font and suitable for printing on an obsolete type of printer ..
There is no irony. In any communication, there are senders and receivers, writers and readers. The senders/writers/publishers choose a medium for communication which their audience is highly likely to be able to receive, interpret and process. The timing of choice of format and protocol for any particular communication depends on the (perception of) the capabilities and preferences of the recipients. While readers of IETF specs might have been expected to be able to read HTML and PDF as well as (ASCII) text, there was resistance until other processing (like 'diff' tools) were also effective. Faxing stayed common until people could reliably send documents in the recipient's language in some other way. It's a matter of consumer cost of receivers. 😉 -- https://LarryMasinter.net https://interlisp.org