Yes P2P CAS (torrent/freenet/ipfs) sounds very interesting over mixnets. Do you have links to specific implementations?

Freenet manages to implement some kind of anonymity without mixnets, relying on inserting/retrieving storage blocks sacrificing latency. In my opinion that's the better approach, because if you separate storage and communication, like ipfs-over-tor, you'll never get anything with proper anonymity and decent performance.

Beyond that, I've heard of xolotl related to gnunet, but I am not sure to what extent it is actually implemented and/or working, as usual with gnunet.

Are you aware of modern Freenet reimplementations, by any chance?

content-based storage is not "instead" of TCP. It's a different solution for a different requirement.

correct, content-addressed storage is often implemented on top of TCP. I wrongly used TCP to generically mean "location-addressed".