So this is basically the insider-like program that Microsoft is running. I wonder if they'll also have slow and fast rings too so people can sign up for the faster updates or the more stable ones. I did not see any specific hardware requirements for devices supported by this program.

The 'fast' ring is surely the releases that are made available to developers - just that they're not free. You need to enrol in the Apple Developer Program for it.

Apple is learning that their recent iOS releases have gone very badly until the first patch or two and is taking steps to fix it. Open betas are nothing new, they're just new to MS and Apple.