Bigger question: MacOS Catalina no longer supports Silverlight at all, because Silverlight is 32 bit only. Is there a workaround?
It appears to be stick with Mojave in some way:
Don't upgrade to Catalina
Use a Virtual Machine to run Mojave in Catalina. This is about the cost of a year's subscription, so not sure how great it is.

How to Run 32-Bit Apps in macOS Catalina
Apple's latest version of macOS, Catalina, officially ditches 32-bit app support, but you can still run the software you need by following these steps.