Same here, just figured out a solution. Use Configurator 2 on another Mac to recover the BridgeOS on iMac back to old production version. You need to reinstall Catalina with macOS Recovery. The reason that Siri is not available is the new T2 firmware I guess, the Big Sur upgrade take the firmware into 18XXXXX, (it’s 17XXXXX in Catalina) I’m not sure wether this is the root cause.
NeilsonG
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