Yes because there are more people playing on Linux than on Mac, and Valve wants Linux to become more popular which is why they made the Steam Deck to begin with. Plus again, it's a lot easier to support Linux than it is macOS since the Linux build uses the same APIs
Correction, Valve started to push Linux with the Steam Machine. They realized no dev was going to waste their time making native Linux versions of their games. They did a course correction in 2018 by developing the Proton layer to help emulate Windows games through Linux. It works well and helps to bridge the wide gap. While the Steam Deck has entered into the conversation, Valve might have missed the boat to effect PC gaming. Companies like ASUS are showing Windows will clap back. Yes, thanks to Proton, it would be much easier to move games from Windows to Linux; with Valve pushing it through themselves. CS is a Valve game, so they had their teams build out both versions together. However, games like RE Village that can play on the Deck, aren’t native and Valve does all the Proton layer testing for the publisher and developer.
And now you know why Valve doesn't support macOS anymore. You admitted yourself you don't play the game, or any game on your Mac. The majority of people on Steam for Mac weren't playing CS, they were playing other things. So it's not worth it in Valve's eye to support macOS anymore, especially given how much Apple makes game development on Mac a nightmare that the majority of gamedevs do not want to bother.
There’s an old MacSurfShop shirt that used to read: “Mac for productivity, Linux for development and Windows for solitaire.” Yes, I did build a PC strictly for games and use my Mac mini for everything else. I used to be into Mac gaming back into the PowerPC days, plus a little bit when I got my iMac late 2012 model. Someone like me admitting I don’t play games on my Mac hardly moves the needle either way. Apple’s long history of being anti-gaming (especially during Jobs time) is still to blame for this one; plus this App Store push with their new Metal tools.
I do agree with a lot of your points. Yes, I get why they did it, but this flies completely in the face of what they were advertising as “PC freedom,” so they choose to support Windows (which is 96% and still growing) and Linux (their project); but Mac need not apply? Drop the choice portion and then it’s all good.
Yes, I’ve been a Steam fan for a long time. Got burned on the Steam Machine and just got rid of my 256GB Deck. It’s best to use Steam as a storefront, whether it’s Windows or Mac. I suspect Microsoft will try to push Steam off Windows at some point too.