MTG Arena doesn't seem to have information available on which cards are legal in its Pauper format. Does this format only allow cards which have had Common rarity in MTG Arena itself? Or is it simply Standard Pauper? I'm not sure if that's the same thing or not—I just started playing Arena recently and am not sure if there has been a set rotation since it began. I have noticed some cards being played at Uncommon rarity but I think they all have Common printings from another set which are still Standard-legal.
Jon N points out that with the Arena Deckbuilder's Advanced filter set to only show commons, some uncommons will nonetheless appear.