At WWDC 2019 Apple announced that Mac apps distributed outside of the Mac App Store would have to be notarized by Apple to run by default on macOS Catalina. The rules were temporarily adjusted in September, but now Apple says these apps have to be notarized by February 3, 2020.
App Notarization
The process of notarization means that Mac developers submit their apps to Apple to make sure they’re safe to install. Notarized apps will pass through macOS Gatekeeper, which checks for malware by making sure apps outside of the Mac App Store have a Developer ID certificate.
If you haven’t yet done so, upload your software to the notary service and review the developer log for warnings. These warnings will become errors starting February 3 and must be fixed in order to have your software notarized. Software notarized before February 3 will continue to run by default on macOS Catalina.
As a reminder, all installer packages must be signed since they may contain executable code. Disk images do not need to be signed, although signing them can help your users verify their contents.
Further Reading
[With Notarization, Apple Moves to Greatly Reduce Malware on Macs]
[macOS Security Will Never Stop Us From Running Software of Our Choice]