Trade Route cares about whether supply piles are full
This is not quite correct. Using ambassador (Seaside), you can refill a supply pile containing a victory card and the trade route won't be affected. The city, however, is affected by this.
(AJD makes clear that he means when a card is gained the first time from that pile. However, I wanted to emphasize that gaining a card the first time from a supply pile and the pile being full is not equivalent!)
One interesting question would be the usage of trader (Hinterlands): You have trader and 2 coins, announce to buy the first estate, reveal trader and get a silver instead. Is the coin token still on?
I think yes, because trader says: "If you
would gain a card [...] gain a silver
instead" and the Prosperty rules make clear that the coin tokens move from the supply piles to the trade route mat as soon as the first victory card is
gained (which it isn't using trader).
Any different thoughts on this?
With watchtower (Seaside), however, you can buy the estate and immediately trash it causing the coin token to be placed onto the trade route mat.