The following quests could be expanded to amenity=shower of which there are around 32k:

AddIsAmenityIndoor:
116 are marked indoor=yes while 273 are marked outdoor=yes.

AddWheelchairAccessBusiness:
1652 have wheelchair=yes and 792 wheelchair=no, though I believe this to be heavily slanted, as wheelchair accessible ones are more likely to be mapped as such. For me the primary issue is that it might not be obvious to the user if a shower is wheelchair accessible, since there are not always signs. Alternatively a new quest, similar to AddWheelchairAccessToilets might be better suited to explain what is required for a wheelchair accessible toilet, and could be expanded to any place with shower=yes.

AddOpeningHours:
Currently used by 2593 showers, of which "24/7" is the most popular with 1648 uses. I would limit this to indoor=yes to ensure that we do not ask for beach showers, which are very rarely signed.

CheckExistence:
I would add it to the 6 year interval, similar to amenity=toilets.