I used it on 12/4 of 12/5. It was super quick. They give you a huge ticket that you are supposed to wear around your neck so no hiding it to cheat to ride with family. It was super quick for me. I would definitely use it again.
It's definitely been on and off, even on the day. When I saw your post yesterday, I checked the app, and Single Rider was no longer listed for Indy.
The last time I rode it Single Rider, we saw the cast members holding the lanyards outside the ride and decided to do it. A little later after we'd already exited and stopped for a snack, we walked past Indy again, checked, and they were no longer offering Single Rider.