There's an easy way to do that: when the browser makes a request to some web site, the server serving it will detect the browser's user agent (which the browser sends with the request to the server) and will check the operating system specified. If it's Windows 10 - don't fulfill the request.
They are talking about trackers, not anything web based. So unless a client announces the underlying operating system there shouldnt be any way to implement such a block.
Sure but chances are that 90% of the users don't know how to do it. Also it's a good warning for the users. It's primarily not for protecting the torrent site but for protecting the users anyway.
I have no idea if the the trackers are doing this or not, but you could use TCP/IP stack fingerprinting. It's a bit costly, so I doubt it's in use. But it could be one way to achieve it. It's certainly not 100% accurate, but my experience with nmap is the detection works surprisingly well:
Here's a random example of user agent detection http://stackoverflow.com/questions/6163350/server-side-brows...