I have written a proposal for adding information about the coins and banknotes that are accepted at a feature, especially vending machines, but in some cases also amenities and shops. I decided to make this an addition to the payment:= namespace and use the value format of the charge=* tag to integrate this as seamless as possible into existing tagging practices. You can find more information in the proposal:
I would not do that, the first proposal is perfectly fine as it is.
Omitting the currency if there is only one local currency might be ok, although I don’t like it - when exactly does it become ambiguous?
Using subtags instead of currency codes in the values makes adding, maintaining and reading information more difficult. You always need to figure out what the local currency might be and check at least to different keys if they exist.
Using payment:notes:EUR=yes even duplicates our currency tag that should be used to name the currencies one can use. This should not be repeated in a new payment key.
This description is in the Wiki since more than 10 years now. It seems that payment:coins:CZK is the only one that has more than 10 use cases, the total seems to be well below 200 in total.
Only for local, unofficial payment methods (“XLT-XXXX”) which are not real currencies the scheme has been adopted.
I would not add such a tag to a new proposal. Keep it limited to the real thing you plan to add (denominations). Side effects always have a negative impact on the acceptance of a proposal.