Cryptocurrencies would be a good solve for this. Specifically, a layer 2 network on top of a cryptocurrency like Bitcoin's lightning network or payment channels

Cryptocurrencies would be a good solve for this. Specifically, a layer 2 network on top of a cryptocurrency like Bitcoin's lightning network or payment channels on Ethereum, both of which allow for subcent transactions with subcent fees. There are obviously UX challenges to making it easy to acquire the crypto, but I could imagine this starting as an optional alternative to captchas.