We thought we were ready...
This morning, having previously notified people that Big Sur was being blocked and deferred, I start work to several messages that it was being offered by the Software Update tool.
We have the 'Defer major updates' config profile in place, scoped to everyone:

but in every case it was there plain as day in the GUI tool, though not listed by softwareupdate -l, which just showed the 10.15.7 supplemental update.
More curiously, despite the fact that it has supposedly been deprecated. I've now successfully run softwareupdate --ignore "macOS Big Sur" on these machines, and it works. The gui tool went back to offering only the supplemental update.
I'm forcing this to run as a 'Files and processes' policy and will follow up once that has had time to spread. Anyone else having similar issues this week?
If it matters: all our macs are MDM enrolled (Apple Business Manager). We're also using a config profile to stop the .app running, but we regard this as a fallback in case the above fails.

