The Epicyon [1] ActivityPub server is now able to be installed on i2p [2] addresses (aka eepsites). This means that it's now possible to run fediverse instances in a purely end-to-end encrypted peer-to-peer mode.
i2p is a bit slower than using a Tor browser, so don't expect rapid page load times. Be prepared for a leisurely pace of events. Get ready to drink coffee and stare out of the window, similar to dialup internet in the 1990s for any older hackers who still remember that. As a practical example, the inbox timeline which takes a couple of seconds to refresh on the clearnet in Firefox might take 10 seconds for an onion instance in a Tor browser or 15-20 seconds or longer on an i2p instance.
Within the deploy directory [3] there is a script called *i2p* which can be used to install an i2p instance on Debian or Arch/Parabola. With some modifications to package names it could be adapted for other distros.
To access i2p sites use an ordinary browser, like Firefox, and within *preferences/network* select *manual proxy* and set http to *127.0.0.1 port 4444* and socks5 to *127.0.0.1 port 4447*. Also turn off DNS via HTTPS if that is enabled. Then your i2p domain name should resolve. Unlike in a Tor browser, when in this proxied mode you won't be able to access clearnet sites, only i2p ones.
When not running on an ordinary domain name there are also limits to federation. Instances on i2p domains will typically only federate with other i2p instances. Familiar instances are typically not set up to use i2p and so won't federate. So on an i2p instance don't expect to be able to federate with mastodon.social.
It really depends on your situation. The obvious advantage is that servers aren't needed and you get end-to-end security without any additional coding effort and without needing to rely upon dodgy certificate authorities or DNS/ICANN. The client/server model does have some advantages though, since the server and its admin provide a rallying point around which to build a community with common interests and shared moderation. This could still happen in the peer-to-peer model though, because there's nothing to stop each peer having more than a single user account. So interesting semi-server hybrid architectures are possible.