It's not non-standard; it's actually in the standard: https://www.rfc-editor.org/rfc/rfc7231#page-24

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."

"Some existing implementations" really just means fetch. The p*ing contest between two groups resulted in a neutered and prescriptive fetch.

In other words, it's fetch that is non-standard, and the actual HTTP standard had to be updated to let you know that.