This is a useful feature added to macOS Catalina. But how to tell if older hardware will support this new feature?
I found a quick way to distinguish it. macOS Catalina seems to use Intel's HEVC hardware encoding. So as long as your old Mac has the 6th generation (Skylake 6xxx Series) or newer Intel processor, it definitely supports this feature.
Blacklist found by Steve Troughton-Smith
Sidecar supports iMac 27" (Late 2015) or newer, MacBook Pro (2016) or newer, mac Mini (2018), Mac Pro (2019), MacBook Air (2018) , MacBook (Early 2016 or newer), and blacklists all the devices in the screenshot.
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Apple stated that Sidecar is part of Catalina so if the hardware supports Catalina then it will support Sidecar.