This is not my understanding of how to triage in MCI. ATLS definitely does teach the importance of deprioritizing people that will be hard or impossible to resuscitate in favor of prioritizing people that leads to a roughly utilitarian greatest good.

That said, I have never heard "black tag = not breathing = don't try to help"[1]. I know for a fact that the boundary for who to try to save immediately and who to deprioritize is very grey in many cases (it is why you should have an experienced doctor doing it). A common example why that equivalency is not taught in ATLS is the case of a boat capsizing in frozen waters. There may be many frozen victims who are not breathing but could still be very salvageable medically. If you chose to not help those who are not breathing in that case you would plausibly be found negligent without some extraordinary situation to justify yourself with.

Another example would be mass overdose on fentanyl, many are saveable but none would be breathing when you first see them.

1. https://en.wikipedia.org/wiki/Triage#Conventional_classifica...