In material instances with the parent material depicted in the attached image I expected "Param" to be considered as unused since it's attached to Opacity and it's not used (ie not attached to output pins). However Param is still listed in instances: if I move the mouse on it the suggestion states "Found in: None".
I tried various combinations of Set/Make/Get/Break material attribures and other unused attributes.
upd. Regression was checked on //UE4/Release-4.26 CL 15973114, issue was reproduced. Regression - No
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-124658 in the post.