I don't know the number, but if there is class action law suit, one thing is certain these people aren't happy with iPhone 4S's performance with iOS 9.
Are memebr of class action lawsuit only people who aren't happy? Certainly not, there are fair share of people who aren't happy with iOS 9 but didn't say anything.
Now, what I don't get is why you so against choice? If people are happy with iOS, they certainly won't downgrade. If it is really small number of people aren't happy, then their downgrade won't cause a dent to iOS. If large number of people aren't happy, then Apple has problem. What Apple did is the most lazy way and most effective way to push people buy new hardware.
Now, with iOS's adoption rate slowing down, banning people downgrade won't help adoption rate, it will only deter people update.