The guys in the Catalina unsupported thread figured it out how to enable Airdrop on unsupported machines but I just can't recommend to the OP fiddling with the kext replacements
Unlike your iPhone, when you AirDrop file or pictures to your Mac, they all go to the same place - your Downloads folder.Choose a recipient from the AirDrop sheet Make sure both the Mac and the iPhone (iDevice) are on the same WiFi network This tip is not an AirDrop requirement, but our readers tell us this often fixes problems between devices If your router sends out both 2.4GHz and 5GHz on the same SSID, check that you connect to the same bandwidth as wel Use AirDrop on your Mac - Apple Suppor Choose AirDrop from the sharing options listed. Or Control-click the file in the Finder, then choose Share from the shortcut menu.
Open the file that you want to send, then click Share button in the app window.