Title text:
Many things about Star Wars were not well planned out, but having a 37-year-old in old-age makeup play the Emperor in Return of the Jedi was such an incredible call.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3157/
I can’t find a film date, but McDiarmid was filmed for the special edition of Empire Strikes Back as the hologram of Palpatine. If you want to be exhaustive, the actor (voice acting or otherwise) also has canon appearances as the character in Rebels, Obi-Wan Kenobi, Tales of the Jedi, and The Bad Batch.
Edit: Oh, how could I forget? The Exegol Broadcast as heard in the Fortnite X Star Wars event is considered canon, and the Fortnite fan wiki cites McDiarmid as the voice actor (I genuinely could not find a better source).