The Package Manager setting (menu: Edit > Project Settings, then the Package Manager category) lets you Enable Preview Packages for your current project. By default this setting is disabled because you should only use preview packages for testing or providing feedback to the package developer. For more information about the preview state and the lifecycle of packages, see Package states and lifecycle.
When enabled, preview packages appear in the All packages filter view in the Package Manager window.
2020.1에 Enable Preview Packages 추가됨NewIn20201