There’s a noble history within OSM of people saying “mappers will never want to map X”. “Non-notable trees” was an example, and when people started mapping trees beyond “notable” ones confusion ensued because the people who had mapped “notable” ones couldn’t tell which ones were “notable” any more. This, I suspect, falls into the same category.
I’m not a user of these things but have mapped the things sticking out of the ground that you can connect a car to. Near me they tend to serve 2 car parking spaces (so each one has capacity=2) and, now that there are more of them, they tend to occur together. The “thing sticking out of the ground” is analogous to a petrol pump (which I’ve never bothered mapping in garage forecourts, but some people do) and the “places where they occur together” is analogous to “amenity=fuel” - the refueling area of the garage forecourt.
If some people are using “amenity=charging_station” for the “thing sticking out of the ground” and some for the “places where they occur together”, then people like me, who try and make maps from this data, are going to need some way of telling the two apart. As Simon noted above it isn’t “capacity”, because each “thing sticking out of the ground” might have a capacity of >1 - as I said, near me, it’s often 2. It also surely isn’t the object type within OSM (see my earlier comment).
Anything related to physical appearance is probably out because of the variance between different examples of the same sort of thing - I’m saying “thing sticking out of the ground” and Simon’s saying “wallboxes”, so neither of those is ideal. I really don’t care what we end up using, just that it needs to not be confusing and not already in use for something else.