Run Sidecar with older Macs

One of macOS Catalina’s major new features, in conjunction with the latest iPadOS, is Sidecar. The ability to mirror or extend your Mac’s display on to your iPad has several practical applications. Mirroring the desktop provides an extra display that can be useful if, say, your family are trying to gather around your screen; extending it provides additional screen real-estate, including the ability to run one app full-screen on your main display while being able to access another, with a suitable Apple Pencil, from your iPad.

But there’s a caveat: your Mac may be running Catalina and your iPad may be running iPadOS 13, but Apple has arbitrarily then you’re scuppered, right?