There are some German cities (Munich) where you can’t enter the city center with a diesel car that doesn’t meet the EURO 4 standards. EURO 4 is a low bar but there’s really nothing stopping them from eventually implementing it more widely and upping the requirement to EURO 5, 6, etc.

I've been driving a 1996 VW diesel van in Germany including Munich, and nowhere anyone ever actually cared about the lack of the sticker. And now, at 30 years of age, it turned "oldtimer", so it is officially exempted.

Same in Lisbon, nobody cares.