I don't think it is a good idea to put an "in games using this..." effect on a card that drastically changes the power level of some (but not most) cards, including the card itself, because it makes pricing the card very difficult, as showcased with:
Action - 2$ - ?
+1 Action
+1$
Trash a card from your hand.
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In games using this, whenever you trash a card, you may draw a card
If you price it at two and it is the only trasher in the kingdom, then you have nearly a junk-dealer for 2$ instead of 5$, which messes with the balance of the game. Junk dealer probably doesn't break the game at 2$, but that's just an example. On the other hand, if Junk dealer is in the kingdom, then Junk dealer is strictly better than this, so it should cost less. What do you do, then, what price do you give to this card?
If I slapped the effect "In games using this, whenever you reveal a card, you might trash or discard it" on scout, I would be against pricing that card at 4$ (as overpriced as scout already is), even though if sage is also in the kingdom it would severely outclass it.
So what you want is an effect that either:
a) doesn't apply to the card itself
b) only applies to the card itself (and then why not put the effect on the card itself if possible?)
c) applies to all (or at least most) cards equally.
So that the card can be priced fairly.
Note that the only official card using the exact phrasing "In games using this, ..." is duchess, every other card discusse here actually reads "setup: ...". Interesting points to consider are embargo and trade route, since they add rules to the game that are in effect even if no example of those cards are currently visible (gaining curses, moving coin tokens around). So a cost-modifying effect would be alright for example, since by the time the pile runs out, most players should have internalized already that the cards' prices are different than usually. Same with Young Witch and the bane, you are expected to remember what card was the bane even though it's not written anywhere.