http://paste.idk.i2p/TardiveTermite/read
I know full well of all the various attacks that can prove you are part of the I2P network, but all of them take place inside the network by fellow routers. But, the question was about if his ISP could tell he was specifically joining I2P, and his response is essentially “They can see that you are using encryption and trying to hide, but they cant specifically see what protocol or tools you are using, or what you are doing”.