After I fresh installed Mojave, when I generate a new SSH key it uses the new OpenSSH protocol 2 format private key.
ID_RSA file starts with "BEGIN OPENSSH PRIVATE KEY", even if I run
ssh-keygen -t rsa -C "firstname.lastname@example.org"
According to the ssh-keygen main page, the BEGIN OPENSSH PRIVATE KEY file type is generated by specifying either the -o command line option or creating an ed25519 key type.
How can I create a private key using the old format?