Hi ***80,
Thanks for coming to Apple Support Communities for help with Siri. It sounds like you were using Siri in Notification Center previously.
This should still be available in Big Sur. Have a look here: How to use Siri on your Mac and the section "Keep Siri results where you can find them".
If you still don't see the option, try disabling Siri inside System Preferences and restarting your Mac. Activate Siri again and see if the feature is back.
If it isn't, do you see the feature inside a new user account? Here's how to make one: Set up users, guests, and groups on Mac.
Let us know how it goes.
Have a fantastic day!