Useful comment about git using the ECDSA key instead and running into a mismatch issue (from the Github discussion):

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After I followed the instructions to remove the old RSA key, `git pull` started using the ECDSA key, and now shows this warning:

    Warning: the ECDSA host key for 'github.com' differs from the key for the IP address '20.205.243.166'
    Offending key for IP in /home/forge/.ssh/known_hosts:87
    Matching host key in /home/forge/.ssh/known_hosts:88
    Are you sure you want to continue connecting (yes/no)?

In this case, I think the old ECDSA key for the github.com IP needs to be removed from `known_hosts`. This can be done with:

    ssh-keygen -f ~/.ssh/known_hosts -R 20.205.243.166

It worked for me, hope this helps.

I'm not sure why the ECDSA key of the github.com IP has supposedly changed - maybe someone can clarify this?