Complicated with NAT and IPv4/IPv6 mismatches.

Not sure why this is downvoted, DCC is direct connection which bypasses the server, p2p.

Likely because it needs a IRC server to set up the handshake, and the exposure of IP addresses. Never mind that DCC and NAT don't mix well.

P2P is impossible without exposure of your IP address, for hopefully obvious reasons.

No different than hitting a webserver. IRC networks cloak your IP address.

I agree about NAT, but that doesn't negate the fact that it was peer to peer that worked for over a decade.