Apparently you can get a mobo with switchable BIOS config (or was it just a switchable SSD?) so the OS didn't even know that there's a second OS around. If there's no connection of the other OS then MS can't break it [as easily]!
IMO it must be malicious, because otherwise it would be caught with remedial testing. I can't believe MS don't include dual boot setups in their testing.
The functionality is present on my new Lenovo laptop, various generations of HP elite/pro books/desks, old asus mobo and newer cheap gigabyte mobo, 7th gen intel nuc.
This description doesn't really do it justice. ~75% of top 100 games work well out of the box/with minimal tinkering according to https://www.protondb.com/dashboard (it varies a bit based on the rating scale)
Many work perfectly and many work even better than they do on Windows. Valve's work really changed the game over the past few years.