Here is the best way around this "Nag" if you, like I, can not or are unwilling to rush to Big Sur...
Step 1: Remove Garage Band from your Dock if it is there.
Step 2: From Finder->Applications: Drag Garage Band to the Trash (NO NOT EMPTY!!!!) (It has to be the Trash, not Desktop, etc.)
Step 3: Reboot
Step 4: Confirm the update for GB is gone from App Store
Step 5: Drag Garage Band out of the Trash and back to Applications
---- this might be good enough, but I did the following to hopefully insure long term persistence ---
Step 6: From Finder->Applications: Rename Garage Band to something like "GB 10"
Step 7: Add GB back to your Dock if you like it there.
Hope this helps. :-)
Dear Apple: Please do not show Application updates for Applications that are not compatible with the running OS. Some of us are professionals that need stability and can't always rush over to the latest greatest. -Thanks!