Other than Coinbase, are there reputable exchanges that provide unauthenticated access to prices over IPv6 (ie compatible with HTTP outcalls)?

[THLO]

Yes, there are others, for example:

• Kucoin (https://api.kucoin.com/)
• OKX (https://okx.com/api/v5/)

Note that the exchange rate canister currently uses Coinbase and these two.