It was possible to do this with:
sudo softwareupdate --ignore "macOS Big Sur"
That would hide the "Upgrade Now" button BUT:
First they removed that ability in macOS Catalina 10.15.5:
Major new releases of macOS are no longer hidden when using the softwareupdate(8) command with the --ignore flag
And then they added this in macOS Catalina 10.15.6:
Major new releases of macOS can be hidden when using the softwareupdate(8) command with the --ignore flag, if the Mac is enrolled in Apple School Manager, Apple Business Manager, or a user-approved MDM.
So unless your Mac is enrolled in MDM (and even then it seems to be buggy) there is no longer an official way to hide this function.