https://wccftech.com/which-mac-and-ipad-models-are-compatible-with-sidecar-list/

Here is a list of all the Mac and iPad models which are compatible to work with macOS Catalina’s Sidecar feature.

While many of you have already decided to dig right in, I’m pretty certain some users are playing it safe at this point. I’m talking about those who are somewhat reluctant to upgrade to macOS Catalina, and maybe wondering whether or not a certain feature would work right out of the box. One of those features is Sidecar, which turns your iPad into a secondary display for your Mac running macOS Catalina.