I never thought he was World Class, said that at the time, so it's not that unusual to me no.
Look at Torres, his decline from one of the most lethal Premier League strikers to misser of open nets probably happened quicker.
Ter Stegen, who is having a good season at Barca and is considered world class, has conceded 81 goals in 85 games for them. Bravo conceded 44 goals in 75 games.
Sticking just to La Liga, Bravo let in 41 goals in 69 games (.594 goals per game), and kept 39 clean sheets (56.52% of games). Meanwhile, Ter Stegen has conceded
31 goals in 36 games (.861 goals per game) and kept only 15 clean sheets (41.66% of games).
Bravo was a world class keeper by any metric. Why did things go sour at City? A contributing factor has to be his lack of English, as on numerous occasions, including against Hull, he has been found his defense inappropriately positioned for him to deal with threats. The rest is mentality, he needs a break, perhaps a fresh start elsewhere, and a chance to find his old form.