Sounds like tech could be used to increase safety. How about a mask with emergency oxygen that detects when you pass out and administers the oxygen? Maybe location beacons similar to tacking undersea animals?
In freediving, minimalism and freedom are emphasized, so additional gear runs contrary to the central ethos.
Regarding masks, the mouth and often nose are not covered to enhance the mammalian diving reflex. Many divers forgo goggles as well.
Freediving is also a very decentralized sport, so it is unclear how safety standards could even be implemented outside of the very niche competitive setting.
In comparison, SCUBA has a certification system and means of enforcement via tank, trip, and service providers.
That is, people could end up pushing even further knowing they have the safety net but blacking out before they get to use it.
Yeah it wouldn't be easy, but you could maybe use an EEG lead, read diver mouth/eye/face state via cameras built into the mask, or something else. I wouldn't be surprised if someone with more physiological knowledge knows of some involuntary signs/actions that occur when one loses consciousness.