I discovered that you must have a BIOS profile that is showing your hack as in the range of computers that will be updatable. so even though I used SWU Switcher Apple wouldn't let me see Big Sur because me computer looked like it could not install it.
I had a profile that was for iMac early 2013 and I could not see Big Sur.
when I changed it to the correct range then I could see Big Sur and also I see the beta version that was made available the other day in the App store.
Time to try is install.