Is it weird that this wasn’t in a TPM somewhere? I don’t know how host keys work, are they rotated based on some root cert regularly or is it just the same keypair since the creation of GitHub?
Given that the key is extremely long lived, this has unfortunate implications: If any of these servers is compromised, or decides to go randomly spewing memory content because of a bitflip, or screws up the nonce on a DSA/ECDSA operation, the key can be compromised. This is hard to exploit if you're a random person, but for a global adversary that collects internet traffic at scale (e.g. NSA), it's feasible and I would be surprised if they weren't exploiting such issues. They were collecting HTTPS handshakes for a reason.
You can't get two (well functioning) TPMs to have the same key. They come with their own, un-extractable* and unchangeable keys built in.
*TPMs claim this, it is probably not impossible to extract keys just incredibly difficult and requiring specialist knowledge.
All cloud vendors offer the same functionality, if you think about it, so it's not an obscure feature.
If anyone is using a Windows machine with TPM-based bitlocker encryption, you have followed the instructions at https://support.microsoft.com/en-us/windows/back-up-your-bit... I hope?