I prefer to kind of look at it in reverse. In the last 22 years, there have been six fatalities in Indycar. It's impossible to 100% say, of course, but the aeroscreen might have helped prevent a fatality for any accident where the death was caused in part or wholly by a solid object hitting the driver's head (Wilson, Wheldon, Moore).

More to the point, it probably would have helped prevent non-fatal injuries due to head strikes, like that suffered by Hinchcliffe in the 2014 Indy Grand Prix. It might give extra protection in a crash like Johnny Rutherford in 1980 @ Phoenix, a case where the car flipped hard enough to snap the rollbar (the helmet saved Rutherford's life in this case). And it gives extra protection to crashes where no one was injured in the end, but might have been. EG:Dario Franchitti @ Michigan 2007 where his car flips and comes down on top of Scott Dixon's car, and AJ Foyt IV's helmet was reportedly also hit by a tire. Or Aleshin burying his car under Montoya at Toronto in 2014.

Yeah, "saved lives" for every crash where the aeroscreen deflects debris / cars / etc. is hyperbole, but that's what happens with any bit of safety technology. Hyperbole happened with the HANS and SAFER barrier too. It's hard to prove whether these "saved lives" at an individual crash level, to be honest. But at *minimum* these technologies prevented some serious injuries, probably even saving a life or two along the way. I look at the aeroscreen in the same way, so I kind of forgive the exaggerations.

Not Wheldon sadly, the entire top half of the car was ground off in that crash and cheese grated against the barrier

Not Moore either, the rotation of the car meant his head would still have impacted the concrete

Don't forget it's largely there to deflect Debris and tyres, there's only so much force it can handle, accidents like Wheldon's, Moore's and Bianchi's were just not survivable in open wheel cars, Halo or not.