Not defined means, it could be anything. If accepting body in GET is out of spec, then spec is supposed to say, GET cannot send body.
If there was an utter ban, then it would be against specification and not compliant, not merely out of specification.
That's not what it means at all. Being out of spec means the spec explicitly stating that a request with a body should be reject. If the spec does not state that a request with a body should be rejected then you are not required to reject a request which packs a body.
No, not defined means it's not within the purview of the spec. Spec doesn't care. You can send one. Maybe it'll work, maybe it won't, maybe it'll crash, maybe it'll be rejected, maybe some proxy along the way will strip it and the server won't even get it, maybe it'll get your client banned forever.
All of these are fine, because spec doesn't care.
> If accepting body in GET is out of spec, then spec is supposed to say, GET cannot send body.
No, then it would be against spec, like HEAD with a response body.
Yes it does. "No defined semantics" = "out of spec".
> I can roll out a service with endpoints that require GET with request bodies and it would still be valid HTTP.
You're out of the HTTP spec entirely.