Example: my fave MP3 player, XMMS, uses GTK 1.2.10 toolkit that no distro (that I use) ships anymore.
Certainly I can build the toolkit, but doing so amounts pretty much to building Gnome 2, and that's really no fun at all.
So, yeah, in theory there are no deprecated environments on Linux. In practice however...
I think *NIX use of C & C++ as main system & library level languages is a great thing - the languages are defined by ISO or such - can't be arbitrarily modified by single vendor to counter perceived competitive threat.