Fedora 28, dual NVIDIA 1080 GTX cards
Broken: 2.79b f4dc9f9d68b
The Maintain Volume constraint appears to have no effect when combined with an IK constraint. I created a stretchy leg rig for a character in Blender 2.78 following this tutorial (https://www.youtube.com/watch?v=m-ErKAsL4QE) a while ago, and when I upgraded to Blender 2.79 the rig no longer held its volume like it did in 2.78.
The constraint works on its own, but if it's combined with the stretch IK features it appears to have no effect anymore. I'm assuming it's a bug but it could have also been a change by design.
Exact steps for others to reproduce the error
Open attached blend file and move the foot controller around. No errors or warnings thrown in the console.