I've heard of Windows build leaks several times, and there were lots of interesting builds. However it's pretty rare to hear about an internal pre-beta build of macOS that made it out of Apple. For example I know there were Leopard builds leaked back in the day that looked like a mix of Tiger and Leopard, as they were transitioning away from the former's Brushed metal design. Also it would be awesome to see an early macOS Big Sur build leak with design elements from Catalina and earlier still in it.