I hope we all can at least agree on the second point : that far more people understand the term Era of Progress, 1875-1965 than do they the term Era of Modernity 1875-1965.
Now onto the first point.
All the many and varied postmodern -isms have at least one key element in common : they involve hitherto verboten mixing.
Be it of high and low (say in art) or normal and abnormal (say in sexual cum civil rights) or central/official/dominant or local/periphery/subculture (as in imperial/hegemony relationships).
I used high and low as my first example deliberately because the Era of Modernity was also the era of Social Darwin defined Progress.
Deductive Progress : absolute universals in an absolutely closed universe yielded absolutely correct results
Everything and everyone was slotted in one universal and eternal vertical hierarchy.
In/out, normal/abnormal etc were really just variants of the all important high/low distinction in worth.
The elements of that ladder were never allowed to touch, let alone intermingle : for it was a heavily modified 'survival of the fittest' version of the old Great Chain of Being, only now leaning forward forty five degrees.
Supposedly 'Iron' Laws of Nature : absolute determinism and absolute reductionism
To the bottom left, were the oldest, smallest, weakest, stupidest beings destined - by 'the iron laws of nature', to quote a highly typical modernity cum progressive phrase - to be replaced by the newer, bigger, more complex and smarter beings at the top right - Civilized Man.
(Man as in, definitely not a woman.)
Any sort of mixing of any of the rungs of this vertical ladder or chain of 'worthiness to continued life' was definitely not allowed.
Instead, being half breeds, mixed breeds, sterile and defective hybrids, engaging in miscegenation, being of mixed bloods and 'passing' were Progress's most extreme crimes.
Now it is well known that most civilized people in the Era of Progress regarded the advanced civilization on Earth, when measuring both culture and technology, was Germany.
As is well known, the horrific postwar evidence coming out of Auschwitz immediately put paid to the exalted status of eighty million Germans as the most civilized and the most progressive nation on earth.
In a much slower fashion, it also put paid to the two billion of the rest of us in our easy lazy unexamined notions of exactly what was civilized and progressive behavior.
In 1945, we suddenly had the top of the Great Chain of Being being revealed as cold blooded mass murderers of humanity, something usually described as the behavior of life at the bottom.
Meanwhile, the bottom of the Great Chain had also become the mass lifesavers of humanity in the form of fungus made penicillin, something you would normally more expect from advanced German chemists at the top than from primitive penicillium slime at the very bottom.
1945 : Progress upended and mixed - the low high and the high low.
In a sense, it was a return to an earlier way of thinking, when it was assumed and accepted that good and bad intermingled in every being, in every society, in every time.
Added to this - a new feature - was a growing understanding that all lifeforms had to intermingle to keep the biosphere running smoothly - that global commensality was a fact, not a choice.
If Art today is a pastiche, so too is Life.
Life does not always imitate Art : sometimes Art, post 1945 Art as an clear example, rushes forth in catchup mode, trying to imitate Life ....