I believe providing a body with GET is non-standard, which could lead to problems with proxies. IETF is introducing the QUERY method to fill this gap.
In standard HTTP/1.1, any method can have a request body. In Representational State Transfer (REST) as defined by Dr. Fielding, HTTP doesn't even come up, let alone "methods" per se, so there is no distinction between DELETE, POST, or GET from a REST standpoint, only within HTTP as an engine for hypertext. Further, in HTTP, any of these requests can contain a request body.
But, because of this behavior by the WhatWG for Fetch, the IETF has added this paragraph to the specification for HTTP/1.1:
"A payload within a GET request message has no defined semantics; sending a payload body on a GET request might cause some existing implementations to reject the request."
In other words, it's fetch that is non-standard, and the actual HTTP standard had to be updated to let you know that.
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.