Twitch has made it possible to include a message with a re-subscription notification in an update on June 20th. Just like before, you have the option to display a highlighted notification showing off your resub in chat but now you can attach a personalized message.
You’re allowed to send one message per month to each channel for which you have a subscription. These messages are limited to 255 characters and can include emotes. If you’d prefer not to leave a message, you can dismiss the feature entirely.
For those streamers concerned about the possible content of those messages, they can be moderated and will adhere to banned words lists. Additionally, alerts provider you’re using have, or may soon have, options to remove or preview these messages before they appear on your stream’s overlay.
For further information and answers to some common questions about the new feature, you can visit the FAQ page.