Microsoft has announced Edge Preview builds based on Chromium will be available in early 2019 for testing, they’re still committed to that, but they’ve not revealed any details will it available for access for general public, on the other side, they’ve opened Edge insider program page and invited developers to sign up to get notified when the preview builds are available, this may create some confusion that Edge preview build will be available only to insiders for testing.
Seems that won’t be case and you don’t need to install Windows insider build to test or use it. Microsoft Edge’s Senior Program Manager for Edge, Kylealden confirmed in a Reddit thread that Microsoft Edge that uses Blink engine will be available as a separate download.
When a user asked is there a possibility of making Edge Preview build available in March 2019 with Skip ahead ring to insiders, which he feels is the correct place to introduce new features/functions, Kylealden said ” We can’t commit to specific timing just yet, but we do plan to make this available as separate download so you can install whether or not you’re configured for Windows insider builds.”

Some users don’t want to mess their systems by using unstable Windows 10 insider builds and by offering Edge preview build to the public, more users will be able to test and report issues.
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