"Reject with message" disappeared from the incoming call screen on Samsung S2 Plus?
The "Reject call with message" disappeared since I turned off the "Messaging" app (i.e., the standard SMS app) because I was using a different SMS app (Textra). 

Re-enabling "Messaging" the "Reject call with message" is back.

Maybe this trick can be used if you want to disable the "Reject call with message" feature.