From comments being posted on articles here, there’s still some confusion over whether macOS 10.15 Catalina will allow you to install and run old apps which aren’t notarized, or new ones which aren’t either. To clear this up, I’ve diagrammed the whole process in detail, to show you how you can work with Catalina’s new security rules.
You can of course just double-click a quarantined installer or app, and see what happens. In that case, you’re just jumping down a few steps, which in the great majority of cases should be fine, and successful. But here’s the detailed sequence that should ensure there are no surprises.
The first question is whether the app, or its installer, is going to try to install a kernel extension. Although this is unusual, it makes a very big difference in Mojave and Catalina, as kernel extensions now undergo stringent checks before they can be successfully installed. The simple answer here is to ensure that you install a recent version which is fully notarized and compatible with Catalina. You can still install some kernel extensions which were signed before April 2019, but that can also get complex. And if a kernel extension signed from then on hasn’t been properly notarized, Catalina is probably not going to install it at all.
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