[brimble]

I really, really wish Apple would release an official way to run arbitrary older versions of their OS in VMs. They have a hypervisor and have for a long time, but using it to run an older OSX/macOS version is very much a DIY thing. It'd be hugely helpful for people supporting software, who need to test on older versions and maybe even to occasionally compile an important bug fix for something that's now "ancient".

[Wowfunhappy]

If you have an Intel Mac, VMWare Fusion works really well for this! I have VMs of Lion, Mountain Lion, High Sierra, and Big Sur available at my fingertips within VMware Fusion 8 running atop Mavericks.

I do all of my development with VMs. It lets me instantly switch between xCode versions, and install dependencies without messing with the state of my real system.

(P.S. I also have Tiger and Snow Leopard VMs, but this requires patching VMWare and is definitely not endorsed by Apple!)