Sidecar, which lets you use an iPad as a second screen, will only support a limited number of machines. If your Mac is getting a little long in the tooth, you’re probably going to be out of luck.
It seems that a strangely small list of Macs are officially compatible with Sidecar:
• 27-inch iMac (Late 2015 or newer)
• iMac Pro
• MacBook Pro (2016 or newer)
• MacBook Air (2018)
• 12-inch MacBook (early 2016 or newer)
• Mac mini (2018)
• Mac Pro (2019)
If your Mac isn’t on the list of supported machines, don’t be too disappointed just yet. [Developer Steve] Troughton-Smith has also posted a workaround that may get Sidecar working on unsupported devices — but it’s not guaranteed to work.
To enable Sidecar on older Macs and devices (sadly, doesn't work on mine): defaults write https://t.co/LLWkWTxkE0.sidecar.display allowAllDevices -bool YES
— Steve Troughton-Smith (@stroughtonsmith) June 5, 2019
MacDailyNews Take: The workaround also might work on some unsupported Macs, but not as well as Apple determines it should.
Of course, were early in the beta period, so Apple could officially load more Macs into the Sidecar by the time macOS Catalina is released this fall.