Amnesia is DRM free after the 1.3 update... on Windows only. If I try to launch it on Mac 10.8 as a standalone app, it errors out with "Could not init Steam." Uncertain about Linux though.
It MIGHT be possible to downgrade Steam (though I don't know how), but you could also update the OS to 10.11 El Capitan for free. Though, I don't think the HPL2 editors are supported on anything above 10.9 Mavericks. You'll have to decide for yourself. I'd recommend 10.8 Mountain Lion, but I do believe you must pay for it. But as an upgrade I don't think that payment is very much, compared to a clean OS install. Might be around 30 USD or so, but I'm not sure.
Silly how it would just stop working for 10.6 though. I hope that's just an error on Steam's part that will be fixed.
Edit: Apparently
ML is 20 dollars. Definitely not the best option here, but a consideration.