That's not true, Coinbase has active USDC/USDT, USDC/EUR, and USDC/GBP pairs. I've traded on them overnight. They just disabled the 1-1 conversion with USD.
My bad. Looks like they added those pairs in 2021 and I didn't notice (though it looks like you have to "opt out of the USD and USDC unified experience" in order to access them). I edited my comment accordingly.