I have been developing a Safari Web Extension and I am pretty new to macOS app development. I have signed my app with a Developer ID certificate still Safari asks me to Allow Unsigned Extensions for my extension to show up in extensions tab.
Distribution to end-users for Safari Web Extensions is limited to the Mac App Store. You can test Safari Web Extensions distributed with a notarized app by using the Allow Unsigned Extensions option from the Develop Menu.
The bottom line is that Safari Web Extensions must be distributed via the AppStore.