The new Safari Technology Preview update is available to both MacOS Mojave and MacOS Catalina, the latest version of the Mac operating system introduced at the June Worldwide Developers Conference.
The Safari Technology Preview update is available through the software update mechanism in the Mac App Store for anyone who has downloaded the browser. Full release notes for the update are available on the Safari Technology Preview website.
Apple's goal with Safari Technology Preview is to gather feedback from developers and users about the browser development process. Safari Technology Preview can run side by side with the existing Safari browser, and while it is designed for developers, it does not require a developer account to download.