On postmodernism and identity politics
Postmodernism arose directly from a chain of subjectivist idealist trends of thought, such as existentialism as articulated by Jean Paul Sartre in the years after World War II and the structuralism inspired by Ferdinand de Saussure and followed by Louis Althusser and others.
Socialism: Utopian and scientific
As background, let me cite the fact that in the Communist Manifesto of 1848, Marx and Engels had already differentiated Scientific Socialism from Utopian Socialism of three kinds: