I could see insurance acceptance being a useful attribute of less mainstream practices such as urgent care and walk-in clinics, as well as optometrists. It could be even more relevant in some other countries.
There are already 88 occurrences of payment:insurance=*. You don’t need to go through the proposal process to extend the payment method scheme; new payment methods get added all the time. Some subkeys like payment:credit_cards=* are generic, so yes wouldn’t necessarily mean that every insurance plan would be accepted. I think this would address the main concern that befell the previous proposal without needing to get into too much detail.
Further subkeys would be possible, but as with the payment scheme in general, I would intuitively expect us to focus on superficially observable payment methods, without attempting to be comprehensive. This may limit the utility to researchers, but a consumer application could indicate (any) insurance acceptance alongside out-of-pocket payment methods, to provide some balance.