Seriously, half of the episode is Vader doing or saying amazing things. And by amazing, I mean very evil, violent, powerful things. Whether it's in the past or the present of the show. I don't think there was any digital de-aging. It looked like make up only. I really don't mind these things at all. Like, at all. It had to be shown. So you find a way and show it. Art is about ideas and concepts, not about worrying about these things.