Could the non-smoking sign still be turned off in modern airplanes?
It would appear that the aircraft you flew was D-AGWR, a A319 with MSN (Manufacturer Serial Number) 4285 delivered in April 2010 (hence fairly new).



There is a switch at the bottom of the overhead panel, next to the seat belt sign, slightly right of the middle. Flip it and they will extinguish. The functionality is a bit more complicated than it would appear, since it feeds into the central aircraft electronics. 



Olivier Cleynen at Wikicommons

Out of interest, this button no longer exists on the Boeing B787, suggesting that it is on the way out. Only the seat belt option remains.

The slots for no smoking are being increasingly 'redeveloped' into a sign for electronic devices instead on some types. They just put a no smoking sticker on the seat in front of you to fulfil the same function. I do not know if they cockpit text has been modified on the aircraft that undergo this modification.



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