Or what if they're not warning you anymore?
I don't know what the EU laws are, but I have rented numerous cars from Sixt, Europcar, Hertz, Avis, Budget, and more for the past five years - and none of them ever had warning stickers. Yes, even Obereluxus - Benz S320CDI, BMW 740d, etc.
Maybe in the past they did, but not anymore. You can always try asking.

Are you planning on taking the car to Russia?