One of the key features of macOS Catalina is Sidecar, which allows users to use iPad as the secondary screen of Mac, directly impact other third-party software (such as Duet Display or Luna Display).
However, Duet Display has not given up. Today, Duet Air is launched to extend the secondary screen support from iPad to any Mac computer.
If users have multiple old Macs, Duet Air can become your secondary screen, and the official said that it will have the smoothest experience and the lowest latency. Next year, Dust Diaplay will continue to add a lot of new features and change algorithms to improve performance.