Sure, you can find some, but thousands of examples? I think it’s far more often the case that the players that look to have gotten worse really aren’t, they are just being expected to play at a level beyond their abilities so they look bad. For example a player having a great season in the Championship, getting sold to a PL team and doing poorly most likely isn’t receding from their peak, they were never a PL level player to begin with.
Rashford and AWB examples are fluctuations in form, an inherent lack of consistency, and/or fit with the manager’s tactics and what the squad needs. This is different than an age-related decline in quality when a player hits a point where their experience and knowledge of the game is no longer able to make up for the loss of athleticism.
Outside of significant injuries, it’s rare that a good player hits their peak in their early-mid 20s and can never get back to that. A lot will reach their potential and stop developing but they don’t go backwards. That’s why it’s so surprising and attracts attention when it does happen, e.g. Dele.
A one off season is not their actual level, it’s just being in great form…