One thing missing from the hypermedia section: links do not have to live in the payload. RFC 8288 lets you carry the same relations in a Link header, and for some clients that is the only place they look. I advertise a markdown twin of a page as Link: </llms.txt>; rel="describedby"; type="text/markdown" for exactly that reason, no body parsing needed.

Conditional requests feel like the other omission. ETag and 304 are part of the same "use HTTP as intended" argument, and they change client behaviour more often than whether someone hardcoded a route.