I don't want to require API keys (or payment) for read access to public data. It is acceptable to not provide support unless you pay, and to allow more request

I don't want to require API keys (or payment) for read access to public data. It is acceptable to not provide support unless you pay, and to allow more requests in a time period if you sign up. (This seems to be the case for the exchange rate API mentioned in another comment, so that is OK.)