I think this is about what is required. To get the benefit of being signed (i.e opening without user interaction to get past Gatekeeper) will in the future require being notarised.
So you will still be able to ship an unsigned app, but you won't be able to ship a signed, un-notarised one.
Apple's website actually says that they will:
"Note that in an upcoming release of macOS, Gatekeeper will require Developer ID signed software to be notarized by Apple."