If I remember correctly, during November/December 2018 all Stream Moderators, Community Moderators and Global Moderators had the green MOD tag. It was visible next to their usernames on their comments in the streams where they were Owner or Moderator.
Sometime around January 2019, the MOD tag for Stream Moderators became black, the MOD tag for Global Moderators became purple.
How did it get changed? Aliens of course👽👽👽