Why did the lead singer of AC/DC quit?

I'm curious to know, can you explain to me why the iconic lead singer of AC/DC, Brian Johnson, decided to step down from his position within the band? What were the reasons behind his departure, and how did it affect the group and their fans? I'm particularly interested in understanding the health issues that were reported as contributing factors to his decision. Additionally, has the band been able to adapt and continue making music since his departure?