18 months after the above posts I am trying this too - long story but because of certain CAD software reverse compatability issues it would be convenient for us to have Big Sur on this M1 Max MB Pro just to overlap with a few more previous versions - so we made a bootable Big Sur install USB - got as far as the screen where one picks the destination drive but the installer will not allow the the M1 Max drive to be selected, an apparent dead end.
My sense from multiple sources that date back to the early days of the silicon chips Big Sur could at one time be installed on the M1 Max but seemingly no longer, perhaps for one of the reasons mentioned above. Still looking for a hack tho, because I don't like these kind of unexplained cul-de-sacs, or assumptions either, as per mi7chy...
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