It's truly ridiculous
An application that uses binary plugins has to be signed and notarized, but doesn't have to require the plugins to be signed, let alone notarized.
*Old* versions of an application suddenly needs to be notarized, even though they were already signed, unless they were installed before Catalina.
Notarization takes an "unknown" length of time and checks for a continually-changing set of things, with an output that's machine-formatted but not machine-readable.
At the time of writing, some of the requirements mean it's impossible for an application to support older versions of macOS - meaning users have to choose between staying on an old version of the application, or losing some of their older software.
Testing this is an utter nightmare.