I've tried to mirror an "Action Constraint" on Piero's "Crown Feather Bones" in Blender 2.90:
1) First I selected all mirrored bones on the right side:
2) Then, I opened the "Action Editor" within the "Dope Sheet Editor" and opened the "Action" for spreading the "Crown Feathers":
3) After this I set a "Rotation Keyframe" on "Frame 1" for the selected right side bones. This created new channels within the previously loaded "Action":
4) Finally, I copied the keyframes from one of the original (unmirrored) bones to the channels for the right side bones:
The bones on the right side of the "Crown feathers" now follow the "Action Constraint" but they only rotate half the angle towards the head and the old bones now move too far and only stop inside Piero's head:
The "Crown Feathers" movement with the original "Action Constraint":
Rig is here.