It quite clearly started after his disaster showing at the 2018 World Cup. Hasn't been remotely the same since. His last two seasons have actually been a slight improvement over the two seasons before that, so it's hardly him slowly declining. He fell of an cliff pretty much overnight at that World Cup.
17/18 - His absolute shot-stopping peak, making saves left right and centre that he had no right to make. That then led into the 2018 WC where he shit the bed for Spain and it seemed to utterly destroy his confidence that he's never recovered.
18/19 - Average most of the season. It was a very noticeable decline and his worst season since his first here, but he wasn't too bad and had so much credit in the bank that we weren't too worried. But then he utterly collapsed in the last couple months of the season with career-worst form, and was the main reason we missed the top four.
19/20 - Poor all season. Ole bought him into the line-up at the semi-final stage of all the three cups that season, where we lost all three with him badly at fault for two of those.
20//21 - Poor all season. Was directly at fault for the goal that ultimately knocked us out of the CL. Even Ole ultimately dropped him because of his poor form, but bought him back for the Europa matches where he was somewhat at fault for the final loss.
21/22 - Actually quite good again for the first half of the season. The only time in these last five years that he looked remotely like his old self. However he dropped right back off in the second half of the season and was fairly poor.
22/23 - Some good, some bad, with the trend being slowly downhill the longer the season wore on.
So not only has he been average-to-poor for most of the last five years, but he consistently has poor performances in the bigger games when the pressure is on.