This might be not 100% correct as some cards that got downshifted to common, e.g. Gut Shot, Elvish Vanguard, etc. wont be downloaded by checking the rarity.

A Pauper download preset should also fetch all variants from all sets. Do we have legality information for cards available?

A Pauper download preset should also fetch all variants from all sets. Do we have legality information for cards available?

Looks like it!

I note that the ScryfallImageSource has logic to fetch and use Scryfall's bulk data dumps using the all-cards data set;

    private static final String SCRYFALL_BULK_FILES_DATABASE_SOURCE_API = "https://api.scryfall.com/bulk-data/all-cards"; // 300 MB in zip

The card objects represented within have legalities fields available, which include things being Pauper legal (amongst other formats). We'd effectively need to just filter on .legalities.pauper == "legal" to determine which art items to download, taking into account rarity downshifting and banlists etc.