The Reaping affix was enabled for Mythic Keystone dungeons on the 8.1.5 PTR about 5 hours ago. Please post your feedback here:
EU:
https://eu.forums.blizzard.com/en/wow/t/feedback-reaping-affix/19104/
NA:
https://us.forums.blizzard.com/en/wow/t/feedback-reaping-affix/69256