I posted this some weeks ago, but it didn't catch up. But I've been thinking about it for some time.
Did we concede more shots against us in the period when he was good? Because the stats in total are about the same.
Year - games/conceded goals:
19/20 - 43/44 (15 clean sheets)
18/19 - 47/63 (10 clean sheets)
17/18 - 46/35 (22 clean sheets)
16/17 - 45/40 (16 clean sheets)
15/16 - 49/48 (19 clean sheets)
14/15 - 43/43 (14 clean sheets)
13/14 - 52/55 (20 clean sheets)
12/13 - 41/42 (12 clean sheets)
So apart from 18/19 being horrible and 17/18 being brilliant, the games to goals ratio is in pretty good balance. So is he getting worse but conceding fewer shots, so the stats don't show it? And if this is the case, could it all come down to concentration lapse when being unoccupied prior to chances?
Here are various stats regarding De Gea but they are done by competition:
https://fbref.com/en/players/7ba6d84e/David-de-Gea
His save percentage in the PL was 78.4% in his first season at United, then 77.2%, 69.1%, 71.7%, 70.3%, 75.7%, 80.7%, 71.6%, 73.6%
The stats for other competitions are available on the link I mentioned above, also the number of shots on target and the number of saves. I imagine it would be possible to add the various competitions together to get the number of shots on target De Gea faced during each season, to find the answer to your question. On closer inspection, they don't appear to have stats for domestic Cup competitions for the early De Gea years.
2019/20 - 150 shots on target (all competitions - 43 games), 44 goals conceded, 109 saves
2018/9 - 200 (PL + CL - 47 games), 63 goals conceded, 147 saves (I'm surprised that the numbers appear to indicate a lot of own goals this particular season)
2017/8 - 169 (PL + CL + FA - 45 games), 33 goals conceded, 140 saves
2016/7 - 129 (PL + EL + EFL only - 43 games), 40 goals conceded, 90 saves
2015/6 - 157 (PL + CL + EL only - 42 games), 43 goals conceded, 114 saves
2014/5 - 127 (PL only - 37 games), 36 goals conceded, 91 saves
2013/4 - 170 (PL + CL only - 47 games), 52 goals conceded, 118 saves
2012/3 - 136 shots on target (PL + CL only - 35 games), 34 goals conceded, 102 saves
2011/2 - 168 (PL + CL + EL only - 37 games), 43 goals conceded, 125 saves
(apologies if I've made any errors calculating the above figures)
From other calculations I have done, De Gea has been an average (10th to 12th) PL keeper for the last 2 seasons. Fbref (link above) also provide stats for sweeper keeper and dealing with crosses, so you can get a sense of an individual keeper's approach to the role. Compared to other PL keepers, De Gea makes a minimal attempt to act as a sweeper keeper and deal with crosses.
I suspect that De Gea's inner confidence took a huge hit due to his errors during the World Cup. Since then the stress of the situation has got to him far more than it used to.