Realistically, what are you expecting? If Valve suddenly decided tomorrow to release all of their source code on Github, all you’d get is a big blob of source code that is purpose built for Valve themselves and not really modular. They’d have so much technical debt and auditing requirements that it’d probably be easier to start from scratch, which I don’t think it’s reasonable to expect them to do.
And honestly, nothing closed source that Steam does is really novel enough to warrant being open source. The value of Steam comes from its ecosystem and playerbase, as well as the backing of Valve themselves. That’s not something that an open source Steam server or client would allow people to compete with.
I would like them to release an open source command line tool for downloading, launching and DRM-validating-ing games though. That seems reasonable for people who don’t want to run the full client and want something like Heroic or Lutris to be able to hook into.