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]
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.
I'm not sure if you realized, but you are not any kind of intellectual authority in any matter related to art. The idea that all artists are leftist is just imbecile, to say the least.
You are just part - probably only a satellite - of the self pleasing circle of modern art critics that ultimately live on the backs of the working man.
People that write stuff like this about a urinal on a wall:
"Arensberg had referred to a 'lovely form' and it does not take much stretching of the imagination to see in the upside-down urinal's gently flowing curves the veiled head of a classic Renaissance Madonna or a seated Buddha or, perhaps more to the point, one of Brâncuși's polished erotic forms."
I don't think all artists are leftists, but I do believe that all art that has a broad impact is progressive in character, if not in intent.
Did the Renaissance painters who used single-point-perspective do so in service of seizing the means of production? Obviously not. But they did do so knowing full well that it was a break from the previous art styles, and that such a break was a good thing. Likewise for the rise of the novel, the fall of meter in poetry, digital and electronic art, etc etc
At the end of the day, one of the better (although still incomplete) differentiators between art and simply craft is the deliberate pursuit of novelty of expression. That is, an artist is always trying to say something new, or say something in a new way. You can be exceptionally good at producing the exact same painting, but that doesn't make you an artist, simply a craftsman (and there is nothing wrong with being a craftsman).
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.
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.