Forcing Fusion 11 users to pay after upgrading to Big Sur seems to be a very shady policy.
I've purchased the license and was able to use my virtual machine for only a few months. And I didn't get any warning about this.
Now I need to use a free version? Did I just lose my money? This is very disrespectful.
If it was within the courtesy upgrade period, you can get Fusion 12 pro for free. If it's for personal use, Fusion 12 player is available for free.
As far as warning goes, Fusion always has been highly dependent on OS versions - blame apple for the changes, not VMWare. Big Sur in particular made massive changes under the hood that required a complete rebuild of the application.
Check the Technology Guarantee information in the Fusion 12 announcement, that would get you a v12 Pro license (depending on when you bought v11): https://blogs.vmware.com/teamfusion/2020/08/announcing-fusion-12-and-workstation-16.html
The other option is to use v12 Player for free (assuming non-commercial use)