You've got the chronology and causality wrong. The Fetch API came after the RFC 7230 advice. Due to arguably dubious interpretation of arguably poor wording in

You've got the chronology and causality wrong. The Fetch API came after the RFC 7230 advice. Due to arguably dubious interpretation of arguably poor wording in RFC 2616 (from 1999) that suggested you SHOULD ignore GET bodies, various caching and proxy servers would ignore or reject GET request bodies, so that it became dangerous to use them. Since then, each iteration of the HTTP specs has strengthened the advice. The most recent 9110 family says you SHOULD NOT use GET request bodies unless you have confirmed in some way that they'll work, because otherwise you can't trust they'll work. Fetch was going along with this consensus, not causing the problem. The pool was muddied; nay, poisoned. And so the solution is the QUERY method. That's how things tend to work in such a space. See also 307 because of 302 being misimplemented.