How does that work? Please, I need to know!
I believe it is still open, sign up here:
https://insider.windows.com/
You'll be beta testing the latest versions of Windows but you can go on the "slow" loop so you generally only update to the stable versions every 3+ months. This could also be disabled via some Windows Update trickery. Once signed up you'll be given a download link to get the latest ISO
Then you have a free copy of Windows 10! But Microsoft will be collecting some background data such as what UI elements you use, how often you use notifications and that type of thing. I'm not sure how much data they will be collecting these days but in the early preview they were tracking how often you used the start menu and certain apps.
All of this is confidential but it dpends how much you care about your privacy! I don't give a damn to be honest as they aren't tracking any personal details just usability of the OS.