YoYoGames have announced in an update to their GameMaker Studio 2 FAQ that the beta for GMS2 for Mac OS X will be coming out in the near future. No exact date has been given yet. But their announcement also mentions that the Mac edition will be available to users of GMS2 who have already purchased a license for the Windows edition, at no extra charge. In other words, purchasing a license will entitle you to run GMS2 on whichever platform you prefer, and you don’t need to decide that at purchase time, and you can change your mind at any time, or even switch between OS X and Windows boxes.
This seems to align with the GMS2 license model, which ties your GMS2 license to your YoYo Account, not your computer.
I think this is great for longtime users who wish to get off of Windows PCs, and are willing to switch to Mac. I only wish that YoYoGames had plans to port GMS2 to Linux, as had briefly been floated back in 2014.