We were not able to view the .ICS file in Notes 11.0.1.x on Mac Big Sur. This behavior did not occur in Notes 9.0.1.x.
As shown in the screenshot, this should be enabled in order for us to add it on our calendar.
The documentation below described that the underlying software that view the attachment in Notes 11.0.1 has been changed. Kindly support .ICS file again in Notes 11.0.1.x.