OpenStreetMap <provides> an API interface to their Software and Database.
OpenStreetMap is an Organization.
It provides things. It has things.
One thing is Software. OpenStreeMap Maps and DB.
Another thing is the Software provides an API interface to retrieve data from that Software.
Looks like Wikidata does NOT currently provide a nice proper property to say "provides API".
another view would be to say that the API doesn't "use" a Protocol, but instead that a user communicates with an API through a Protocol.
I would avoid using subclass ... you could say "part of" ... but then it gets lost in the shuffle with Sparql queries because there no easy way to see just API Providers versus Parts of Organizations which could be anything.
It would be much better and way cooler to easily list and see all the organizations that "provide API" and even then filter on those API's by different criteria and categories.