No matter how low the ppO2 limited is dialed there will still be the occasional unexplained death.

OxTox has a time and dosage based limit. It's not about picking a particular number and calling that "safe". As dives get longer ppO2 needs to drop. 1.6 on a 15 minute dive is less risky then 1.4 on a 90 minute dive. On a long enough dive 0.5 is probably the ppO2 limit (just based on O2 tollerance if not Tox)

We are at a point where ppO2 is still an issue but other factors are probably becoming hidden contributors in the OxTox risk. CO2, physical stresses (cold tired), drugs, other gases in the mix or released from materials hoses, regulators - maybe even what one ate.

Some of the OxTox blamed deaths have been nowhere near any time-dose limit anyone has proposed following. Far enough to call into doubt that OxTox was actually the cause.

People still convulse and die walking around on the surface breathing air for reasons completely unrelated to OxTox. Until someone comes up with a new theory to explain mysterious diving deaths OxTox will be blamed for the majority of these events - until long after it has ceased to be the problem.