macOS Catalina's Sidecar feature is limited to newer Mac models
Only newer Mac models support Sidecar, an upcoming feature of MacOS Catalina which gives you the ability to use the iPad as a secondary display to the Mac.
By Cookie Monster -
Sidecar in action. <br>Image source: Apple
macOS Catalina's upcoming feature, Sidecar is supported only on newer Mac models.
Developer Steve Troughton-Smith uncovered the information while looking through macOS Catalina's code. He found out that there is a long list of older Mac models which are blacklisted from supporting Sidecar. The Mac devices listed below are able to use Sidecar:
Late 2017 27-inch iMac or newer
2017 iMac Pro
Mid 2016 MacBook Pro or newer
Late 2018 Mac mini or newer
Late 2018 MacBook Air or newer
Early 2016 MacBook or newer
2019 Mac Pro
Troughton-Smith claims that some older Mac models can still use Sidecar via the Terminal command provided by him, but there is currently no complete list on which older Mac models the Terminal command works with.
As for the iPad, Sidecar should be supported on the iPad Air 2 and later, iPad mini 4 and later, the 5th generation iPad and later, and all iPad Pro models since these models are eligible for the iPadOS update. iPadOS is required for Sidecar to function on the iPad.