Currently, you have three options to distribute software on macOS.
1) You register as an Apple Developer (which costs $100 a year) and distribute through the Mac App Store, where Apple will do their review/automated scans/whatever, resign the app with their certificate, and then distribute it.
2) You register as an Apple Developer but don't distribute through the app store, and instead sign your software with the certificate you get through the developer program. It looks like, starting in macOS 10.14.5, you'll have to upload your software to Apple to be "notarized" if you want to distribute this way. It sounds like it's only for new software at first, but eventually old software will stop working unless it's notarized.
3) You don't register as an Apple Developer, and you just build and distribute software however you like. Currently, if a user tries to run your software it simply won't run, and the user has to know to go into system settings and manually give it permission. What isn't clear (at least to me) is weather this option will remain unchanged. This sentence:
> In a future version of macOS, notarization will be required by default for all software.
kind of makes it sound like like all software will have to be notarized, which implies that you have to be an Apple Developer to distribute at all. But saying "by default" makes it seem like there's some kind of option given to the user, so maybe it just means that software that's distributed by a registered Apple Developer but isn't notarized just moves down into the third tier of software that has to be explicitly allowed to run by the user.
It seems like Apple's ideal model is that if a user wants to run software of any kind, it has to go through them (like iOS). I don't think that's what they're announcing for macOS, but it's a little hard to say for sure.