This is not true, for client keys ssh-rsa is just fine. If you're using certificates then you may need to get those resigned. But most people aren't using cert

This is not true, for client keys ssh-rsa is just fine. If you're using certificates then you may need to get those resigned. But most people aren't using certificates, just bare rsa keys. And ssh-rsa client keys don't specify a hashing algo at all. Even if you specified `-t rsa-sha2-512` it would still output a ssh-rsa .pub.