In 2015, Twitch added alerts to celebrate subscription anniversaries with your community.
When Subscribers renew their channel subscription, they will have the option of sending a personal message in chat for the Partner to see!
The custom re-subscription feature makes anniversary alerts a little more personal. This message will show up in chat which means it is visible to all viewers, and may also show via any enabled third party notification services.
The article explains what custom re-subscription messages are, how they work, and how to use them.