De Gea's mistakes in the latter part of 18/19 (which is still the worst form of his career) was the biggest reason we missed top four. Not the only reason, but if I'm asked to give the biggest reason it was certainly De Gea's form. In 19/20 he was at fault for three of the five goals that we conceded in the Europa and FA Cup semi's. In 20/21 he literally made himself as small as possible to avoid any contact where he might get hurt for the goal that ultimately knocked us out of the CL. That season he also didn't save a single penalty then missed his own in the Europa final. I wouldn't say that he was the reason we lost that one, but let's be honest it's more likely to happen to De Gea than most as he's so weak in penalties. Now this season he was at least somewhat at fault (I'd say the most at fault) for all three goals that Sevilla scored to knock us out of the Europa. He's also had some terrible errors against the likes of Liverpool, City and Arsenal in the league during these seasons. We all saw what happened to him in the 2018 World Cup where he was atrocious for Spain.
De Gea's worst games and biggest mistakes have regularly tended to come in the bigger games over that period. I'm astonished anyone thinks otherwise. Not only in the bigger games as he's also made a lot of mistakes in 'lesser' matches over these five years, but it's definitely been an unfortunate trend.