It hasn't been overnight, his decline has been pretty steady to be honest. Check out his stats for the last four years. (Ranking among first choice PL goalkeepers only, out of 20)
| 2017-2018 | 2018-2019 | 2019-2020 | 2020-2021 |
Shot Stopping (PSxG-GA) | 2nd | 8th | 10th | 15th |
Crosses collected | 16th | 17th | 16th | 19th |
Sweeping (coming out early) | 14th | 18th | 17th | 17th |
Errors leading to goals | 0 | 4 (3rd most) | 3 (3rd most) | 1 (Joint 5th) |
De Gea is still able to pull off outstanding saves that are good for YouTube and easy to talk about in post-match. Its similar for his mistakes, when they happen they tend to stand out. As a result, arguments about De Gea tend to focus on "this amazing save" vs "that terrible mistake" with fans minutely dissecting each to try and prove a point. In truth the occasional outstanding save or terrible howler are not the real issue.
Three or four mistakes a season might seem bad, but you can live with them if a goalkeeper is otherwise outstanding. Nick Pope for example has made more mistakes leading to goals than De Gea (2 vs 1). But Nick Pope is the best shot stopper, best cross taker and 2nd best sweeper in the country. Based on xG, he's saved Burnley 8 goals that De Gea would have conceded. So mistakes can be a red herring when looking at a goalie. The real issue is that his fundamentals have gone to pot. So far this season, De Gea is among the worst keepers in the league on the basics.
This hasn't happened overnight. All that's really happened is that the fans have kind of accepted that De Gea is world class from back when he was, so they only really question that when high profile mistakes come along. Then when a good performance comes along, they tend to think that the mistakes must have been a fluke and he's back to normal. Slowly though, fans have noticed that the high profile saves seem to happen less and less, and begun to look at his performances more closely. And when they've looked at those performances, they're realised there's a bigger problem there than just the odd howler or two.