It was a suspension part that inflicted what was acknowledged to be the fatal injury. A HALO might have bounced it off upwards...
But also downwards....
I think lots of things working together on todays cars would have saved Senna's life.
Firstly the wheel tethers are the most likely saving grace as broken suspension parts generally stay attached to the wheel or the rest of the chassis and can't get anywhere near the driver's head anymore.
Suspension components are now carbon which shatter into superlight splinters in an accident and could not pierce a helmet. Steel suspension of the 90s became heavy sishkebob spears when they broke free from the car.
Senna's helmet very likely was forced up against the wall with the offending suspension parts during the accident. Today with high cockpit sides, improved side impact structures and HANS, the drivers head is more or less kept centralised on the cockpit and away from debris in the impact zone. Watch Mika Hakkinens Adelaide crash to se how drivers heads actually went outside the cockpit in the old low sided cockpits.
Finally, helmet standards have improved massively. The part that the suspension pierced was reinforced via regulations after Massa's spring strike incident. The shell's pircing resistence today is also, via regulations, much more resistent than helmets of the 90s.
The halo is then the unknown. I'm convinced the above improvements would have saved Senna but if somehow a small piece of debris was to aimed at a drivers head its 50-50 as to whether the halo would deflect it or let it through.
For big stuff like wheels and large body components (which have been the main source of injury and death in open wheel racing in the last 20 years) it's obviously massive improvement.