Steam Machines will not support a suspend/resume feature similar to that which we see on the PlayStation 4 and Xbox One anytime in the near future, if ever.
The issue is rooted, according to a Valve engineer posting on GitHub as JohnV, not in the suspend part of the equation. That works fine. It’s in the resume part. When a Linux session is resumed following suspend, the way it rediscovers hardware doesn’t work in a way that works with resuming games. In this case, that applies to the controllers plugged in.
Slashdot user tlambert explains that, upon resume, Linux doesn’t reattach devices in the same way as Windows. Linux doesn’t “reattach devices to the same point in the device tree that they were when the system was suspended,” which can cause controllers to become unresponsive. Multiple controllers can also come into conflict on resume, acting as a single device.
Because Steam Machines use SteamOS, which is based on Linux and meant to be kept open in nature, this probably doesn’t look like something that will get fixed unless it gets fixed at a level more basic than SteamOS.
With this feature present on both the Xbox One and PlayStation 4, it could become a small hurdle for the Steam Machine product line, as many gamers are getting used to it being there.