Did anyone figure out how MITM attacks are handled? How does the key transparency work and does it replace public key fingerprints?
The reason why it is missing (but seemingly planned in the future) is because it is not as critical as this change. This change prevents attackers from recording conversations now and decrypting them when (in the next ?? years/decades) they get access to an actually powerful quantum computer. On the other hand, you can do MITM only after you factorized RSA key (or solved discrete log).
The additional reason I presume is that this typically requires a change to the whole public key infrastructure (certificates, OCSP, etc.) which is a lot of additional work.
You then can build post quantum future secrecy / key rotation on top of that, by mixing in new key material and it remains secure from MITM, as long as the internal state of the endpoint isn't compromised. The endpoint compromise is outside networked TCB threat model, as such compromise could also be used to exfiltrate long term post-quantum identity keys for undetectable MITM).