I am very interested in this as well.
Moreover, I quite often (OK, non that much. It happened three times so far in different areas) to find taps marked as “non drinking water” for some political reasons but all of the local people living there drank that water since years and keeps drinking it daily. The water actually IS potable but is officially marked not as such. How should such a tap be marked? Thanks.
Potable can be subjective or even inconsistent: water can taste badly but be safe for the health or it can be usually ok to drink but the water source may occasionally get polluted so it is not guaranteed to be safe. The city council is responsible for proper marking of water taps. We are not. Therefore I think, we should try to map what the traveller would see in reality. If the source is marked as non-drinking in real life, so it should be in OSM.
The traveller can always ask the locals if in doubt.
However I would suggest sticking to the subject in this thread, since your question actually deserves a separate discussion: when we know something in real life is not true (e.g. for political or commercial reasons), how should we map it? I would suggest asking it at help.openstreetmap.org.