https://blog.cloudflare.com/encrypted-client-hello/
https://blog.cloudflare.com/handshake-encryption-endgame-an-ech-update/
A conference paper about the quantum-safe implementation of HPKE (a basic cryptographic operation in ECH):
https://eprint.iacr.org/2022/414.pdf
https://www.ietf.org/archive/id/draft-ietf-tls-esni-16.html
https://datatracker.ietf.org/doc/html/rfc8446
https://blog.cloudflare.com/rfc-8446-aka-tls-1-3/
Finally there is a roadmap that will be followed during the Hackathon available here