Diffie-Hellman isn't considered to be post-quantum safe: https://en.wikipedia.org/wiki/Shor%27s_algorithm#Feasibility...

> Forward secrecy is designed to prevent the compromise of a long-term secret key from affecting the confidentiality of past conversations. However, forward secrecy cannot defend against a successful cryptanalysis of the underlying ciphers being used, since a cryptanalysis consists of finding a way to decrypt an encrypted message without the key, and forward secrecy only protects keys, not the ciphers themselves.[8] A patient attacker can capture a conversation whose confidentiality is protected through the use of public-key cryptography and wait until the underlying cipher is broken (e.g. large quantum computers could be created which allow the discrete logarithm problem to be computed quickly). This would allow the recovery of old plaintexts even in a system employing forward secrecy.

https://en.wikipedia.org/wiki/Forward_secrecy#Attacks

Perfect forward secrecy doesn't work that well when NSA motto is - store everything now decrypt later. If they intercept the ephemeral key exchange now they can decrypt the message 10 or 50 years later.

Forward secrecy is orthogonal to post-quantum safety.

Perfect forward secrecy requires the exchange of ephemeral keys. If you use either ECC or RSA for this and the traffic is captured a quantum computer will break it.

All perfect forward secrecy means is that you delete your own ephemeral private keys, the public keys stay in the record. And a quantum computer will recover the deleted private keys.

Also, none of the currently accepted post-quantum cryptographic algorithms offer a Diffie-Hellman construction. They use KEM (Key Encapsulation Mechanism).

Not exactly, they can just reverse the entire chain.