It's been about two years since Brian Johnson was forced to quit AC/DC after doctors warned that he faced total hearing loss if he was to continue with the band. Apparently, Johnson's hearing had gotten so bad that he was basically singing from memory, and that a lot of the time he couldn't hear which key the band was playing in on stage.
On a related note, however, I did see AC/DC on the last Pepsi Center stop with Brian, before Axl/DC became a thing, and his singing was solid. I mean, the guy's been shredding his vocal cords for decades, can hardly hear, and still put on a hell of a show. There's something to be said for that. Oh, did I mention he's 70? Anyway, check out what Brian had to say about his ousting here.