By God, you are truly a trifecta of clichés converging.
Where exactly have you got this narrative from? Or even better: please explain how Marxism relates to contemporary art, I can accept just a general line of ideals connecting to each other.
I tried to have some leveled way to see your opinions on my other comments but this went a bridge too far, you seem to be repeating a collage of unrelated stuff, as I said in another comment: it's so bad that it isn't even wrong.
By just about any reasonable accounting, the willful deconstruction of the concept of art reached its peak during and after World War I (e.g. Dadaism), specifically because of the wanton and apparently pointless destruction of an entire generation of French, British, Germans, and Russians (and others besides, but they bore the brunt of it). There was a very widespread questioning of traditional mores, which arguably bolstered the broader Marxist cause, but its definitely an inversion of causality to say that Marxism caused existentialism.
But, Dadaism, for instance, was a far left movement. Its followers, were people that held radical or even far left views. [1]
Marxism is really a cancer that destroys everything it touches. Its final aim was always to destroy everything that is beautiful, elevated or pure about mankind, and we, as a society, have been sponsoring it with our taxpayer money that pays for the self anointed gatekeepers of intellectualism that populate a big part of our Academia - that is to say, all Academia that doesn't get judged by the outcomes of their ideas when applied in practice.
If your definition of leftism is "progressive" then I'm sorry to tell you: all advances in art were made by people you'd consider "leftists".
There is simply no way that a conservative worldview brings any art form forward, not even from classically-inspired backgrounds, by pure definition it attempts to keep the status quo, and all they achieve is a soulless repetition of what art was from the period they considered as "golden".
Not much dissimilar to what you are trying to do, to be honest.
Broadly speaking, all existentialism was leftist in character because its core tenet was a rejection of old ways. Definitionally, you cannot reject the old order without being liberal/progressive/leftist/etc. Which, again, was in response to the 15-20 million killed during WWI, the most deadly 4 years in Europe since the plague years.
Again, I think you're inverting causality by blaming Marxism for post-modernism, when they are instead related results of the same overall trend, that was simply catalyzed by WWI (there's definitely a read on e.g. the 1917 Russian Revolution that it happens at least wildly differently without Tsarist Russia entering the war).
> A sixth, broadly Marxian sort of objection rejects the project of defining art as an unwitting (and confused) expression of a harmful ideology.
But I don't think many serious critiques of "this is not art" claims invoke Marxism. The Marxist perspective generalises the idea that art is incredibly difficult to define, but doesn't originate it.
That is general knowledge, but if you really want to go down that way of "where did you get this narrative from" to try and avoid the subject. Well, you can see it, for instance, in the Stanford Encyclopedia of Philosophy for starters. [1]
[1] https://plato.stanford.edu/entries/art-definition/