Apple is informing developers that all new apps developed using a fresh Developer ID must be notarized to pass through Gatekeeper from macOS 10.14.5 and higher.
Some apps downloaded and installed from the Internet could adversely affect your Mac. Gatekeeper is designed to help protect your Mac from such apps. Apple really wants you to download Mac apps from its Mac App Store.
An Apple support documentation notes that, beginning in macOS 10.14.5, all new or updated kernel extensions and all software from developers new to distributing with Developer ID must be notarized in order to run. In a future version of macOS, notarization will be required by default for all software.