I have been limping along with an old 2009 iMac at work which is unlikely to be updated soon. Increasingly frustrated being stuck at 10.13, I came up with a solution. I downloaded Parallels 16 and managed to install 10.15 on a virtual machine and it runs on my 2009 iMac. It's slow, but not that much slower than 10.13. I was wondering if others had tried this; is this a way to get a couple of more years of use out of an old mac?
You can use Catalina as a virtual machine using Parallels Desktop but there is on 3D Acceleration support.
