Various articles declare that some 25% of coral has died from warming oceans. I cannot determine if that is 25% of particular species, or 25% of all species.
My impression is that it is 25% of reef area. It’s complicated though because when a reef is bleached due to a oceanic “heat wave”, which stresses coral and often leads to death, popular press will have headlines such as “90% of the Great Barrier reef is dead!” and when at least some of that area recovers they will switch to "Don’t worry about the Barrier reef, at least as not as much as we told you last year.
Like the heat waves that cause them (heat in the ocean, that is), bleaching isn’t new, and reefs recover over time, but the frequency, severity and scale of bleaching is increasing, and more and more reef area stays dead permanently as recovery takes longer than the time to the next bleaching.
But because this is a constantly changing state, there is no exact correct percentage of coral that has died.
That may be true but I think its pretty well established that things are trending downward at an alarming rate.