I assume not, but there is a command-line client, steamcmd. I used it the other day when I wasn't near a machine that I had Steam installed on to remotely install FTL on a machine via ssh that I had Steam installed on so that I could copy it to a remote laptop, which I didn't have Steam installed on (FTL is one of the games that doesn't use Steam for DRM)

I'd imagine that one could theoretically slap an open-source frontend on that.

EDIT: Also, graphical frontends aside, it doesn't even have readline/editline support, so running it via:

$ rlwrap steamcmd

...is already throwing a minimal open-source frontend on it that rather improves the experience.

EDIT2: I don't care that much about most of Steam, but I do wish that the downloader portion of Steam were open source so that I could push a patch to let one cap the number of concurrent TCP connections open. Normally, a saturated network connection will tend to allocate bandwidth evenly on a per-TCP-connection basis, and because Steam opens a ton of concurrent connections, it gets the lion's share of the connection...for Steam downloads, which are very much low priority, and which I don't want trying to eat up all the bandwidth.

EDIT3: There's apparently some Linux ncurses-based client that uses library injection to take over the graphical client, but that repo last saw a change 6 years ago, and I'd bet that it hasn't worked in a long time. Looking at steam_injector.c, It looks like it prevents XMapWindow() from running, so it should keep the graphical client from actually doing anything graphical.