A lot of the RFCs are flavored by the lack of https and prevalence of forward proxies run by the ISPs to improve perceived speed and reduce their network loads.. Back in the day.

By my estimation, that's why they explicitly call out only the client and origin can know what a GET body means; proxies should forward as-is and ignore.

Those days of ISP forward http proxies are gone, but those semantics are still fine; the body means what the origin and client agree it means.