Restrict to Windows only

Maybe something like this? - https://kb.fortinet.com/kb/viewContent.do?externalId=FD38671&sliceId=1

I haven't tried it, but I'm guessing if you specify the Windows OS's you want to allow and set this option:

set skip-check-for-unsupported-os disable

Then you'll only be allowing the specified operating system versions.  However, you can only specify Windows versions in the os-check-list command so I'm not sure if it will block non-Windows clients or not.

That's an old KB article, but the commands are still there in my 5.6 Fortigate.