I assume you’re referring to the fact that the coach key never went through the proposal process. However, neither bus nor the even more generic psv ever went through that process either. All three are listed in this tree of access keys that’s pretty well-known among mappers and data consumers.
By the way, there’s also a very common share_taxi key for share taxis (jitneys), which are commonly used as airporters. This key is necessary in some regions where these services are common enough that the authorities reserve stands and parking spaces for them.