Probably not Linux-specific.
I tested with Symphony using the beta_loggin-f_ beta on Win8. Beta tag vanished upon removal.
It happened to like all Linux users who were in the beta channel for Witcher 2 when the channel was removed. Perhaps it is specific to the Linux client then from what MrSchism says.
Oddly enough, have you had it happen with any other title?
It's not too often that I'm in beta channels so for me not seeing this with other games isn't telling me much, but no I haven't.
I've had this happen to me with Hotline Miami and Super Hexagon betas on Linux.
Nothing extracted yet.
If you opt out of a beta, the "beta" notation in the game title is removed, but if the beta channel is removed while you are still in it, the title doesn't get updated. This happened to me and several others with The Witcher 2, when they removed their beta testing channel. Proof here:
http://www.reddit.com/r/Steam/comments/2emsdc/if_a_beta_is_removed_steam_should_automatically/