Cannot update from Catalina to Big Sur on compatible Mac: "The requested version of macOS is not available"
The option to update to Big Sur came back with the release of 11.1.
In the comments an article was mentioned that reported that 11.0.1 might have been intentionally blocked for certain MacBook Pro models because it caused lasting damage (Thank you @Tetsujin &amp; @bmike):

Big Sur 11.0.1 (20B50) was released to block 2013 &amp; 2014 13″ MBPro’s from installing the OS. This move looks to be related to the Big Sur install problems with these models.