Is PI TO PI Interface v. 184.108.40.206 compatible when source PI server is 2016 R2 and Destination PI server is 2015 R2?
There shouldn't be any issues sending data from one PI Server to another with different versions.
To adding Jesse Ru's answer, OSIsoft Tech support's product page contains system requirement.
PI to PI Interface Current Version: 220.127.116.11
Retrieving data ...