In my opinion, that's not on Steam to support their client on a long past EOL operating system. Not withstanding the added development workload and costs, there is also significantly more risk associated with supporting an OS that isn't receiving security patches.
Not to mention the modder's example Windows fucking 98. Steam still supports Windows 7, which was released in 2009. Your 6 year old PC will be fine.
The person is a modder who put DRM in his own mods and is complaining about DRM.