> "right is right, even if nobody thinks it is" is something that people have recognized since ... oh, Socrates?
Something recognized since Socrates, whose death was foretold by Nietzsche, and finally rendered obsolete by the postmodernists a la Baudrillard (1981):
All Western faith and good faith became engaged in this wager on
representation: that a sign could refer to the depth of meaning,
that a sign could be exchanged for meaning and that something
could guarantee this exchange-God of course.
To parse this idea properly it is important to define your terms, and lack of adequate definition is what leads to confusion especially as regards philosophical and especially religious discourse. "God" here should be taken to mean the belief in an absolute truth, a common, shared, objective reality from which our subjective realities are derived; metaphysics, spirits, ghosts, "energies," New Age quantum bullshit, and creationism all need not apply here (and are better dispensed with). It is the underlying reality to which all symbols refer; the concept of the pure simulacrum, the hyperreal, is a symbol with no reference to any underlying reality whatsoever, no referent, and thus untethered to any actual reality - atheistic and "without God."
Language and rational discourse are based upon this principle - that when we exchange words or symbols with one another that refer to a thing, we have some hope in hell of referring to the same thing, such that I can represent an object in the "external world," or even a thought in my intellectual "internal world" or "soul", serialize it into a literate form, and have you deserialize that literate string into the same intellectual or cognitive structures that I beheld in my own soul when I put those words together and hoped to communicate to you.
"The death of God" means that we can no longer guarantee this exchange of "meaning" between minds/souls (both of which I regard as terms that refer to exactly the same thing and are perfectly coextensive), because no symbols are tethered to an underlying objective meaning nor reality. Language and definitions are relative, malleable, subject to revision. Context and intent of language and expressions are irrelevant; the meaning of any string of symbols is now unilaterally determined by their impact or impression on the receiver, and thus any subjective meaning can be inferred from any expression.
The condition of post-truth is a consequence of this rejection of objective reality, of the elevation of relativism to the highest degree, and the confusion of (hyperreal, manufactured) symbols with actual reality. There is no objective truth any more, only relativism, narrative, and belief.