It is now in place. Apple has announced that universal purchases have been active since then on iOS as well as on macOS, watchOS and tvOS. This means that the developers can now share this new feature with their users.
For example, if you make a purchase in an iOS application, you can find it on other systems. For users, this is good news, but developers still need to activate it on their Apple developer accounts. To set up universal purchases, they just use the same bundle identifier on Xcode for each application.
At the same time, Apple has added new categories to the App Store and the Mac App Store. In the case of iOS, we are talking about Graphics and Design and Developers. And in the case of macOS, these are Books, Food and Drink, Newspapers and Magazines, Navigation and Shopping.
Note that developers cannot yet couple Mac applications built with Mac Catalyst with an iOS application that accompanies it using bundle IDs. This means that universal purchases for Catalyst applications are not yet available. Developers should wait for the launch of Xcode 11.4.