The best protection is a machine that is well build:
We have ovens in our kitchens that are designed to reach upto 400°C (for cleaning) and nobody is afraid of them catching fire. Why? They are engineered to be safe.
Similiar a 3D printer that has good engineering is safe and doesn't require an automatic fire extinguisher.
If we talk about low-end China printers then the answer is they might not be as safe but the solution is to fix them instead of adding the fire suppression system.