Yes, I’m still on this rig…
Took me a while to track this down after trying other bone-constraints like the stretch-to constraint which didn’t quite work for me… Instead I got the required results from stretchy 1-chain IK constraints pointing to fixed bones.
Setting the bone IK Stretch setting to 1 with a well-parented 1-chain IK constraint will perform the follow-and-stretch you would expect from a spring… (Eevee test render:)
(Spring-bone targets are parented to the arms here…)