How can we target specifically Windows vs Linux vs Macintosh? This is a feature, right?
At least I can't imagine that such an extremely important and easy to implement (Browser user agent string.) feature does not exist.
But I found no way or documentation of how to activate it. (I did find how to target desktop vs tablet vs mobile, that's not the issue.)
I hope this isn't a magical crass oversight caused by the general notion that advertised software is to be available cross-platform.
You can indeed target by operating system - although these options are only available for Display and Video campaigns.
Keep in mind that you can choose to target only Android, iOS, etc but you are limited to mobile operating systems only and cannot choose targeting for Desktop OS. For more information check out the following article: https://support.google.com/adwords/editor/answer/1211190?hl=en
What you could do is create a smart remarketing list in Analytics based off of operating system and remarket accordingly (although not allowed for RLSA remarketing).
Here's a great article on creating remarketing lists in Google Analytics: https://support.google.com/analytics/answer/2611404