Must we use ssh-agent nowadays?
No, there's no requirement (new or old) to use ssh-agent.
And no, the whole idea of ssh is not that it won't require a password. One of the ideas behind ssh is that it provides an encryption connection between the server and the client.
Determining whether the server and the client are the &quot;right ones&quot; (i.e. the ones you want to allow talking together) can be done through many means. One of them is simply requiring the client to enter a username and a password. If you do not supply a username, the currently logged in user's username will be used instead.
Another way of authenticating yourself to the server is using keys. The tech support agent that stated that you should paste your private key was wrong. Never give out your private key!
Instead you want to paste the public part of the key to the server's interface. This way you can login using the key instead of a username and password.
In order to ensure that others do not misuse your key, it is common practice to protect the key itself with a passphrase (i.e. not username/password, but a &quot;long password&quot;). If you do that, you can either enter the passphrase every time you want to login, or you can use a program like ssh-agent so that you only have to enter the passphrase once when logging in to your computer, and then the key would be ready to use without further passphrases whenever you need to login.
The guide you have followed instructed you to create a config file. You're right that the config file is as such not required, but I won't say that it is not usual practice to have one. The reason why they ask you to create one when following the guide is to have the special instruction &quot;UseKeychain&quot; set. This makes it so that the passphrase protecting the key is stored in your macOS Keychain.
The practical benefit is that now you won't even have to enter the passphrase when you login to your computer. When you login to your computer like normal, that usually unlocks your Keychain as well - including your passphrase.
The fact that you're still asked for a password when logging in seems to indicate that your keys setup is not used. Make sure it is actually the password that you need to enter (and not the passphrase) - if so, that is usually because you haven't properly pasted the public part of the key to their SSH panel.