That would be tagging for the renderer, and we don't do that, you should open an issue on Organic Maps github about this. 

https://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer

Osmand can understand this tagging and shows a non drinkable sign.

There are ~2000 instances of amenity=drinking_water & drinking_water=no worldwide, here is an overpass query if you want to see all of them: https://overpass-turbo.eu/s/1SiJ

amenity=drinking_water does not mean the water is drinkable, historically it meant there is a point where you can get water. This tag is much older than man_made=water_tap (2008 vs 2014)

E.g. on the wiki you can see in the examples that drinking_water=yes is added next to amenity=drinking_water. But on the sidebar it says "Implies drinking_water=yes":

https://wiki.openstreetmap.org/wiki/Tag:amenity%3Ddrinking_water#Photo_examples

So even the wiki contradicts itself, so I don't know. 

I understand your issue, but even if we would change this, OrganicMaps would fail with other 1999 instances, so it wouldn't help much.