My 2014 Mac Book Pro (MBP) has updated to Big Sur without any problems.
APFS FILE SYSTEM AND TIME MACHINE
Thanks to the podcast MacBreak Weekly Episode 740 that referred to an Ars Technica article that suggests that you should re-format your Time Machine drive to the new file system AFPS.
The Ars Technica article explains why you should re-format the drive to AFPS. However, I had a bit trouble in the re-formatting process so I will set out what I did.
When I tried to use Disk Utility on the MBP to erase the Time Machine drive and re-format as AFPS it kept failing and the reason shown was that the drive was in use. This was even after I removed the drive from Time Machine.
So, I attached the drive to a Windows Machine and used Disk Management to delete the partitions and re-format as NTFS. Then I re-attached to the MBP and used Disk Utility to Erase and re-format as AFPS. All went well and I renamed it – Time Machine- !!
I then set up Time Machine again with the re-formatted drive.