Are you referring to this kind of oxygen-blocking mask? Wearing this can simulate a cellar or pit environment, increase diaphragm suction, and enhance the ability to actively absorb oxygen. However, it is not recommended to train for more than 20 minutes, as it may cause carbon dioxide poisoning. Oxygen blocking equipment to simulate plateaus is quite expensive, and ordinary oxygen blocking masks on the market are not as good as imagined.
It is normal to wear a mask when exercising outdoors to protect against smog.