What is a API key?

Hi there, I am new to sentry API, as I have seen from Authentication doc, there is Auth token, API Keys and DSN Authentication. Where I have found where the Auth token and DSN (client key) is generated.

However I can not find any documentation talk about the API Keys at all. According to the documentation, API keys are a legacy means of authenticating. So is it still exist or not? If it still exist, where can I find it?

Much thanks.