In the past, older Mac Pro machines have not been supported by newer macOS releases due to standard features on the machine (e.g. Bluetooth wasn't standard on 2008s, limiting Sierra) or by CPU extensions (SSE 4.2 required for Mojave). With the announcement of Big Sur, Apple states that you must have a 2013 "trash can" Mac Pro or later leaving the older cheese graters behind.
What is it about the 2013 that would prevent Big Sur from running on a 2012 Westmere or even back to a MacPro4,1? One of the benefits of having a Mac Pro was that you could upgrade the Bluetooth and GPU modules and this is how we got (say) a 2008 machine to work all the way up to High Sierra and even Mojave if you were brave enough. But those upgrades aside, I believe the CPUs of a 2012 and 2013 supported the same features so I'm wondering if there is something else?
Best Answer
As to the reason why Apple chose this exact model as the cut-off point for support cannot be answered by anyone but Apple. Everything else is just speculation.
There exists an non-official guide on how to install the Big Sur beta on the older 2010/2012 Mac Pro's (amongst others):
https://parrotgeek.com/bigsur/
Note that because this is non-official, it might not work or exhibit problems. Also it might only work for the current beta, and not when you get to the final release of Big Sur.
As indicated in this guide, the built-in WiFi does not and there could be issues related to sleep mode. This could hint at why Apple does not support this model - i.e. having to support these older WiFi chipsets.