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