Setting up Yubikey hardware backed SSH keys for Linux was a total pain, but setting it up on macOS was actually very simple. Note this is for command line, I’ve not looked at setting this up for GUI applications.
I am assuming that you already have a working GPG key on your Yubiky and want to set it up for SSH login.
First you will need to install GPGTools
Next you will need to set up the gpg-agent config file, add the following lines to the file $HOME/.gnupg/gpg-agent.conf
enable-ssh-support use-standard-socket write-env-file
The final change you need to make is in $HOME/.bash_profile to add these lines:
unset SSH_AUTH_SOCK export SSH_AUTH_SOCK=$HOME/.gnupg/S.gpg-agent.ssh