Well because saying how broken it all is won't really fix anything. I mean ... No one is trying to attack GoG because their games aren't installable from DOS, evet thou some of them would clearly work there. If you are using outdated operating system, you just have to live with the fact that you'll be able to run less and less software on it. It's pretty much implied.
With this kind of thinking, they'd have to keep adjusting Steam for win 2000 for how long ... 2020? I really don't have anything against people who use older OS, it's their own business, but when I was buying games on Steam, I have actually expected they won't support XP by 2010.
OT: Wow, those dates are crazy! I mean, 2010!? In 1998 I thought how everything would be so totally different, bases on moon and crap...

No, not really. What is implied is that you'll be able to use less and less new software on it. Not that software you bought when the system was still supported will stop working.
Actually, they just need to include an easy way for people to keep the program from updating.

Fair enough, that would probably work. Keep in mind, however, that people are able to complain about anything: Including broken games on un-updated Steam. I can certainly see why Valve wouldn't want to do it.

And we have a winner :p
Seriously, THIS is what I meant by "function implies support". Especially if they go out of their way to re-add it. People are going to bitch about anything. If they make you say what OS you are running when you send in a trouble-ticket, we'll get a bunch of "Oh mah gawdz! Steam is discriminating against me because I run an older OS! THey refuse to help me! I am going to go make a thread to rant and then get pissy when people with differing opinions post"

But if you buy a game on steam that supports your OS, but not newer ones. And then steam updates to not support the older OS. How are you supposed to play the game?
By selling a game that works on an older OS, and then making steam later not work on the aforementioned OS, they're preventing users from playing the game they paid for.
It's like if I buy an car that runs on an older type of gas, because I happen to have a gas station next to my house that uses that gas. But then the car company comes round and updates the car to a newer form of gas without asking me - and i don't happen to have a gas station that sells the new popular gas near my house.

And yet, you have agreed that they can do that in Steam's agreement. If you don't like it, what would possibly force you to agree?

Not so much forced as in nobody reads it and were unaware. Standard procedure.

What do you mean? Does it make any difference?

This is my point.
I'm not expecting continued support on an operating system that their data shows only .1% of their user base still uses, as many are assuming in this thread. I just want to still be able to use my games and client at their current level of functionality without being forced to download an update that will lock me out from using my games.
Anyway, the client is still working for now as long as I cancel out the updates before they finish loading. I just hope I don't slip up on that. I also talked about this problem on another forum that was very helpful and suggested a way I might be able to get my Logic Audio program to run on XP, which would allow me to upgrade my OS without losing a very expensive piece of software. So, I should have an option in the future.
Thanks to those of you that offered some helpful solutions instead of just giving me shit for running a system on Windows 2000.

There is an easy way. Offline mode is that easy way.
[i]Offline Mode allows you to play games through Steam without reconnecting to the Steam Network every time you wish to play - this is particularly useful if you do not plan on playing over the internet and would prefer not to download new updates for your single-player games.
Please note that you must connect to the Steam Network and test each of the games you would like to use in Offline Mode at least once to set up your account and configure Offline Mode on your machine.
Using Offline Mode on a PC.
Please follow the instructions below to configure Offline Mode on your PC:
1.Start Steam online - make sure the Remember my password box on the login window is checked
2.Verify that all game files are completely updated - you can see the update status for a game under the Library section (when the game shows as 100% - Ready it is ready to be played in Offline Mode)
3.Launch the game you would like to play offline to verify that there are no further updates to download - shut down the game and return to Steam once you have confirmed that the game can be played
4.Go to Steam > Settings to ensure the Don't save account credentials on this computer option is not selected
5.From the main Steam window, go to the Steam menu and select Go Offline
6.Click Restart in Offline Mode to restart Steam in Offline Mode
[/i]
https://support.steampowered.com/kb_article.php?ref=3160-agcb-2555

Additionally, any new feature they add is potentially a bug. More features, more bugs, and more testing. Every feature costs time, money, and effort. Possibly too much to justify adding it for 0.1% of the customer base.
There's also the issue of stupid customers running a supported operating system turning off the update mechanism and leaving their client open to security vulnerabilities, or possibly unable to download games at all if the download mechanism is changed in the future. You may think the number of people who would do this is very small. I assure you that they exist, and will call or email any available tech support systems at your earliest inconvenience.
From both a software engineering and business standpoint, I'll be very surprised if they add a new feature explicitly for an operating system they've explicitly stated they're no longer supporting.

Yeah, the 32bit version is much more updated and polished.

Could you possibly afford to just get a new computer specifically for gaming and leave your 2k box just to run logic audio? Seems like the most logical (though expensive) solution
Or indeed could you set up a firewall to block steam's ports and only open them when you explicitly need it to have network access?

Looks like it's more like 0.13% according to the Steam Hardware Survey page.
[url=]http://store.steampowered.com/hwsurvey/?platform=pc[/url]
Since the peak number of online users tends to average around 2.5 million, That's roughly 3500 people listed as using a non-XP/Vista/7 version of Windows. Doesn't really seem to be a large enough group to bother supporting any more.

I wonder how many of those aren't even gaming on those systems, but just using it for the IM functionality to keep in touch with steam friends?