Show icon to indicate microphone and/or camera are being used
There’s a product called Oversight that does this. It’s a donation-ware product meaning it’s completely free but donations to support development are encouraged.
It’s accessed through the menu bar, but if something accesses a device, it will post a notification

I’ve used this before for users who who were concerned about malware accessing their camera and it worked well.  I haven’t tested in Catalina, but it should work though (IMO) it’s not necessary due to the security permissions model in Catalina. If you don’t grant an app access to a device, it can’t use it.