Before anything else, take her to your local pokémon center. While it’s unlikely to be related to her injury five years ago (you would have seen plenty of warning signs throughout these past five years if her wing didn’t heal correctly), she may have suffered from rare complications from evolution, or she may have developed arthritis—the latter especially if she was an older pidgey when you found her.
Either way, Nurse Joy may be able to offer treatment solutions for your pidgeot. If the issue stems from evolution complications, your pidgeot may need corrective surgery followed by physical therapy, but over time, she’ll regain full use of her wing.
Arthritis may be more complicated to deal with, and I’m afraid that if it’s this, your pidgeot won’t be able to participate in sky battles anymore. However, with therapy, she may at least return to the battlefield and keep herself in the air for somewhat longer periods of time (depending on her personal condition).