How to tell if my old Mac supports Sidecar feature in macOS Catalina?
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.

HEVC hardware encode on macOS 29:30

 



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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