Is it possible to use a slider toggle from FK to IK, in a armature? I've seen in older Blender character rigs (such as Big Buck Bunny), character armatures had some floating slide toggles that affected whether a rig appendage was Forward-Kinematic or Inverse Kinematic (FK/IK), but newer rigs (such as Sintel) don't.

What is the current "best practice" way of creating an FK/IK toggle (is it part of the core application?), and what version of Blender did that become available?
For the former, it was never version specific and can still be done with later versions, people have desisted from doing so since there are several more efficient ways to do so.
The method I personally use and I think is most people use is a blend of constraints, drivers and a little scripting to create GUI components in the sidebars to toggle from FK &lt;-&gt; IK. I cannot say exactly when this became available but it is a core part of the application.