Here is a USCG letter explaining the situation.
I quote : (my italics)
While the applicability has not changed, placards must be updated to reflect the new garbage
discharge requirements. Until U.S. regulations
are updated to reflect the new garbage discharge
regime and placards are readily available, U.S. flagged ships operating exclusively on domestic
voyages may continue to use existing placards.
U.S. flagged ships on international voyages and visiting ports
of a country that are party to
MARPOL Annex V will need to meet the annex’s placarding requirements to avoid possible port
state control action. Operators are encouraged to make and post copies of the relevant examples
provided in Enclosure (3).
I passed a voluntary inspection
a couple of months ago, with the old placard. The inspector said that the new placards are not yet widely available. He did not have any to hand out.
Therefore I'd take issue with the OP's claim that you could get a fine for not having the updated sticker, at least from the USCG. Other national entities, that is a different matter.