Pay no attention to wich finguer the ball comes off of last. it will always be the middle finger
This is the #1 most overthought & irrelevant issue when it comes to shooting. Your middle finger is your longest finger. The ball will ALWAYS come off of your middle finger last! Only exceptions to this are people with odd hand shape/structure, or 7 footers (and even then, it's rare). In order for the ball to come off of your index finger, you would have to release with your wrist bent at a very uncomfortable angle. I have studied this. I have tested it many times. It's possible to release off ✌🏼simultaneously, but it's not something you should consciously worry about. Just SHOOT.
The question people may be meaning to ask is which finger should they flex the most when shooting. There's no right answer to this, but you are best off focusing on flicking the ball off of your 2 longest and strongest fingers (✌🏼). This is also the strongest portion of your hand.
p.s. A lot of people complain that sometimes the ball comes off their pinky or ring finger last. This simply isn't true. It was just a bad release. If you record yourself in slo-mo, you will see. Get up more reps and you'll have fewer bad releases!