It's always been there when you restart your phone. Maybe you just didn't notice it because you haven't restarted your phone much, or at all. You don't have to worry; Samsung is just letting you know you're on beta software.
As many others suggested, popup is meant to notify that your device is currently on Beta software version and not a complete / official software.
Rest assured, your device will not go on a rampage mode and cause bunch of crashes (we certainly hope not 😭) while on Beta software. Even if it does, please don't hesitate to submit feedback via Samsung Members.
