Assuming there's just the 6 un-trashers (no others in Plunder), there's a 13.4% chance of at least one of them being in a game of all kingdoms full random latest editions (not accounting for Young Witch, Black Market etc.) I think that probability is low enough for the Trash to be considered the trash.

For comparison, when Dark Ages came out, the probability was 10.5% of having either Graverobber or Rogue in the Kingdom. Even just Base (1st edition) and Dark Ages gives a 30.8% chance of un-trashing (not counting Thief).

So fair to assume the trash is the trash when you play a game of Dominion, but of course it depends on the Kingdom.

A card that sends cards back to the box, to "Oblivion" etc. would meaningfully interact with the 6 untrashers, Forager, Necromancer and the "When trash" cards (+Tomb and Sewers). That would actually be quite common as the number of when trash cards is going up. Going to wait until the full set is revealed before calculating probability (and not sure how to include the landscapes).