I know that many people will be interested in Big Sur over the next few months. I obviously don't recommend upgrading to it while it's in beta testing stages. Remember that the developer betas are under NDA so you can't discuss them here at this time anyway. Any posts asking for help with these betas will be removed.
There is good news and bad news about the Macs that retained full support in Big Sur. The good news is that if you chose iMac 15,1 as your SMBIOS, that Haswell refresh Mac is still fully supported. iMac 14,1 14,2 and 14,3 are no longer supported. Those were all first gen Haswell Macs that were released in 2013. Those system definitions are not going to work with Big Sur because all support for iMacs older than 2014 was dropped.
If you are just installing Catalina now and want Big Sur later then choose
15,1 for the option of upgrading your OptiMac to Big Sur later on. iMac 15,1 used a dedicated AMD graphics card. Someone with a dev beta will likely test the GT 710 Kepler card to determine if that works or not. The Late 2013 15" MacBook Pro has a GT 750M which is Kepler GK107. All supported AMD Polaris cards will continue to work. (RX560,570,580 etc.)
The Late 2014 iMac 5K was designated
iMac 15,1 and used Haswell refresh i5s and i7s. This will be the best option for most people that want to upgrade to Big Sur even though they are using iMac 14,2 right now and a first gen Haswell i5 or i7. Changing the SMBIOS will require generating new serials to match this. Take that into consideration before making any changes.