Literally every software company built their business model this way. Go open a support case with any software vendor complaining that their product won't run on Windows 98 and see how many help you out beyond "Buy a computer from this millennium"

No, they didn't. I can install the software I bought back in the day on the computers I bought it for, using the license key provided. GoG also famously uses a model where GoG does not care what OS you're using.

You are failing to understand just how much has changed since Windows 98. It's a completely different environment that requires specialized knowledge to develop for. They can't just dust off some old source code and re-release the client. The entire back-end has changed. It would be a massive undertaking that would appease about 12 people total.

Lol, I'm a software developer that started by writing legacy windows software, I know exactly how much (little) has changed.

Sure, but I would argue that there are a lot of better things that Valve could be doing with those resources than supporting Windows 98

I don't care. They have the resources to support it.

Either strip the DRM out and pay whatever you have to to the publishers to do that, or keep supporting the systems you sold your software for.

The idea that Valve is blameless for shitty behaviour because other tech companies also do that shitty behaviour is nonsense. They have been the dominant platform forever, and have had an insane amount of resources available to them.