I’d argue that Yoda should never have appeared in the PT. Establishing that he was exiled from the order in TPM would’ve been a nice subversion, demonstrating how far the Jedi had fallen since its “golden days.”
Ahh but there’s the real problem, and keeping all of the ESB twists intact would require far more creativity.
Or maybe because it would contradict more lines from the original trilogy? Like Yoda knowing who Luke’s father was enough to say “Powerful Jedi was he” and “Much anger in him… like his father”. How else would he know to “Not underestimate the powers of the Emperor”? And how else would he learn “Wars not make one great” without having experienced that first hand.
That’s another thing that a lot of people miss. When he says “Do not underestimate the powers of the Emperor” that obviously means he faced him before. Obviously he was witness to the Emperor’s power first hand. That actually becomes a plot hole for anyone who removes that duel from the movie, because now how else would he know the Emperor’s power? It fits into the original trilogy perfectly.
In TESB, Yoda seems to know a fair bit about the type of person Luke was growing up on Tatooine despite having never met him in person before the events of the film, implying he’s a fairly strong clairvoyant. If he could’ve observed Luke’s life from afar, there’s no reason he couldn’t have done the same with Anakin, Palpatine, etc. decades earlier.
Also, even if he didn’t fight in the Clone Wars, that doesn’t preclude him from having participated in some earlier warfare in his lifetime. Hell, there’s nothing precluding him from having fought Palpatine somehow, either. Maybe they had a violent Force Skype.