The only open source client that I know that worked with Steam is for bridging Steam's chats and notifications with IRC. It's called Bitlbee.

And i've stopped using it a few years ago because IIRC it was a pain to keep the authentication working with Steam. They thought hackers were accessing my account or something like that, and I kept having to disable security "features" just in order to stay logged in.

I assume an open source client not endorsed by Steam would have the same issue.