The RFCs are clear that a server should ignore any body sent by a client after a GET request header. But RFCs don't matter; real life matters, and in real life there are lots of HTTP clients that can't make a GET request with a body. We had to go into our API and add POST support to every endpoint that was GET + body because there was a long tail of clients that couldn't manage to make the GET request. In practice, support is NOT widespread enough to rely on GET + body requests.
It’s not exhaustive though. Many servers and proxies support it, but not all. For instance, GCP load balancers do not, but ALB does. At least that was case a few years ago.
