There's few deaths caused by MMA, but it's much less than boxing as you can compare it yourself here [1] [2]

I don't think there's a change on boxing to reduce the possibility of death because it just can't. Maybe the referee can make an early stoppage, but most boxers wouldn't agree with that and trying to convince the referee if he can still fight.

Most boxers doesn't realize if they're suffering a brain damage because there's no sign of the disease during the fight, but after few hours they will realize if their body become weak or ill which mostly already too late.

---

[1] https://en.wikipedia.org/wiki/Fatalities_in_mixed_martial_arts_contests
[2] https://en.wikipedia.org/wiki/List_of_deaths_due_to_injuries_sustained_in_boxing