I was pleasantly surprised at this the other day. I was about to jump in to some Friday night games with some buddies when my tower died, dusted off a gen2 i5 laptop running ubuntu and I was playing Payday2 with them within an hour; that Linux filter on my steam library returned a shockingly large list.
As a SC backer, I'm losing hope a bit on the Linux release. They don't really talk about it, I'm not sure they'll have it ready by the time they do release (whenever that is).
I'm not a backer, but I tried to find out about their Linux plans. Linux support was their reached stretch goal, so they should fulfill it. But no one says anything definitive. The last thing I've heard, Chris Roberts mentioned something that they'll make it after the initial release.
They planned to use Vulkan in the engine in general, but it seems that Lumberyard doesn't even support it yet (and Linux is only listed as an upcoming supported OS in Lumberyard FAQ).
If the drivers would just catch up...