Water fountains can frequently be out of service for short periods of time and I'd be reluctant to tag one as disused since it may be working next week. I've pasted the lifecycle section of the water fountain documentation below. If you feel that either disused or abandoned is an appropriate tag please add info to the water fountain poi. 

Link to documentation: https://wiki.openstreetmap.org/wiki/Tag:amenity%3Ddrinking_water


Nonworking features
It is advised to apply lifecycle tagging, i.e. you add a prefix like "disused:" or "abandoned:" to the main key (amenity). This tagging will likely remove them from most maps.

disused:amenity=drinking_water for features that are not active or require only minor repair.
abandoned:amenity=drinking_water abandoned features, these would require extensive repair. Often these have been left for some time as 'disused' before falling to 'abandoned'.