Go see for yourself.

A bit tired, and don't want to go looking at this hour. Does that mean that if I use Arch and send Steam Support a message, they will assist me, or will they tell me "Your OS isn't supported. Unfortunately, we cannot assist you."

It shows. You are asking the wrong questions. Steam does not support titles, the people offering them up for sale, do.

You mean steam doesn't support the steam client, Portal 1+2, Team Fortress 2, Half Life, Left 4 Dead 1+2, Dota 2?

I mean they do not support Borderlands, Trine, etc. The titles that you mentioned are indeed theirs to support. Now, do you have an instance of getting a "sod off" from them because you are using arch linux to run the above mentioned software that you would like to share?

If I had an answer of either way, I wouldn't be asking the question. But my point is, just because you can get something to run on an unsupported system doesn't mean the company selling it will provide support for it. If they claim it's a supported system, they have to provide support. The support will obviously not say "Sod off", but they will just say "Sorry, your system is not supported, nothing we can do to help you."

Oh, and wait, wasn't there a previous example of no support for a title running on different OS? Something about a game not running on XP and higher, while Steam required XP or higher as an OS? Now what was that game...
Will have to look for it tomorrow.

No your point is that getting software to run is not the same as offering support for that software. Cool. I agree with that. My point was that supporting Linux should not be the nightmare that GoG made it to be in that post of theirs and I offered the example of Steam for Linux and associated games running on Arch. Getting software to run is like 99% of the job done in circumstances like these.

Supporting 1 distro isn't the nightmare. Supporting 2 is maybe doable. Supporting more than 3 is a nightmare, especially for a small team. I could offer you quite a few stories of JA2 1.13 support if you'd like, that wouldn't even need to go on the "unsupported systems" territory, and that's for a game we do have access to the source code.
Oh, and about the 99% of the job you mention, how many support requests are about getting the game to run first?
Supporting a platform (properly supporting it, not "It runs on my device") is a bit more pain than most people think, thus the reluctance of some to go there. Unsupported binaries is a different subject though, even if I also dislike the idea of it.

Yeah. I'd rather take my support horror stories from people that are in the trenches, like Ethan Lee, that actually do this type of work (which is porting to and supporting software that runs on Linux). Excuse me if I don't buy into shoddy reasons such as the ones provided in GoGs post. It clearly indicated their lack of knowledge on the matter but played well into the FUD that is repeated on and on in the mainstream media about Linux.

Here's the requirements for titles in the latest Humble Bundle, nowhere do they mention a particular distribution:

http://support.humblebundle.com/customer/portal/articles/1156426-humble-indie-bundle-8-system-requirements

Does HB offer support? I was under the impression that support was provided by the developers/porters themselves.

GOG does support for the products it sells. So if GOG doesn't think it can support it, it will not support it.
Steam is supporting its products on 1 linux distro. All other distros are on their own.
HB doesn't care about distros, because it's not supporting them. If a developer feels confident enough to support all distros, good for him, he will have to support them. If he thinks he can only support 2 distros, he will support those 2 distros only.
You mention the HB requirements, and point that it doesn't limit the distros. I say that it does that because it doesn't offer support, so it's up to the developers what they wish to support. So HB isn't offering linux support, it's offering linux binaries. The developer of said binaries (who may not be the developer of the game, see Dear Esther) is offering the linux support.

In addition, when I was having problems with Bastion, Humble Bundle provided zero help whatsoever.

Fine, so don't support and that is that. Don't insult us with bullshit reasons.

I really have strong doubts that GOG support all the games they sell in the manner that you describe. It's only possible if one gets the sources for each game. Are you saying that GOG get these sources and study each game code to offer full support for game bugs? I highly doubt that, but feel free to prove me wrong. I think in the best case they communicate with the developers about bugs and integrate their patches back to GOG releases. I don't see how it's very different from developers support of Humble Bundle.