Earlier today, Microsoft began rolling out Windows 10 Insider Preview build 18947 to Insiders on all rings, including Fast, Slow, and Release Preview. It was obviously an accident, being pushed in the early hours of the morning to rings that shouldn't be running 20H1.
What's really interesting is that it includes a completely new Start Menu. It finally gets rid of Live Tiles, replacing them with a more standard grid of icons. Interestingly, it doesn't seem to be live for everyone, since our test machine didn't get it.
The Start Menu that we're not seeing is similar to the one that's developed for Windows Lite, the upcoming version of the OS that's made for dual-screen devices and some ultrabooks. It could be that Microsoft is just testing it out and it won't make it into the final 20H1 product, but it's also possible that the Redmond company is aiming for some design parity with Windows Lite and its new Start Menu experience.
If you want to check out the leaked build, it should still be available in all Insider rings. You'll want to act quickly, though, since it will be pulled soon.