Very few people are going to be setting the compile time options for the libraries that implement HTTP/3 to enable plain HTTP/3 (no one has for HTTP/2). So Google, MS, Apple, etc not supporting plain HTTP/3 is a defacto standard more powerful than the spec Google/MS openwashed through the IETF.
Well, the standard doesn't, but the browser(s?) implantation(s) do?:
https://http2.github.io/faq/#does-http2-require-encryption