I asked about something like this previously, the answer from Twitch Development was that they did not desire to have an easy way to determine if someone unfollowed as there is a level of harassment involved when people unfollow channels. That conversation was steered toward finding other ways to determine loyalty.
You can see in this post:
It actually isn’t omitted for performance reasons. @3ventic is on the right track. Unfollow was something the team wanted to implement, but I was very against it for abuse reasons. I don’t want to create a scenario where it is easy for someone to harass someone else for unfollowing. Obviously this can be achieved in other ways, but I don’t want to make it easy.
That isn’t to say that Twitch does not change their mind but, what you propose as an IRCv3 tag makes it very easy to track follows and unfollows.