In Big Sur, the menu bar icon for AirPlay has been replaced with a Screen Mirroring icon.
However, it does not show up in the menu bar by default. To make it show in the menu bar...
Open System Preferences
Click on the icon for "Dock & Menu Bar"
Choose Screen Mirroring on the left. On the right, check to box to "Show in Menu Bar" and choose "always" from the dropdown list.