Until there is a specific Stack Exchange site for SteamOS itself, I think rather than using the subject of the question to decide where the question goes, instead the nature of the question should dictate where the question goes.
For example if you wanted to ask a question about knives, as there is no specific knife Stack Exchange site, you need to figure out where your question is best suited. If you wanted to know how best to sharpen your cooking knife, head to Seasoned Advice, if you were after the best knife for skinning, then the Great Outdoors would be the best place to go, or if you just wanted to know how to replace the blade on a Stanley Knife Home Improvement should be where you ask the question.
Same goes with SteamOS. If it relates to the gaming side of the operating system, ask it on Arqade, but if it relates to the operating system itself, best head over to Unix & Linux SE.