Welche ältere macOS Versionen kann man in Parallels noch installieren und betreiben?
Also ab 10.12.x downwärts?
Neither Leopard nor Snow Leopard are supported to be run on Parallels Desktop virtual machines. That is due to the fact that it is prohibited by Apple's EULA.
For instance, if we look at Parallels Desktop System Requirements, we can see that among non-server Mac operating systems only Lion and Mountain Lion can be installed as guest OS.