The launch daemon (
launchd) is responsible for starting processes at boot, on demand, on schedule, in response to incoming network port connections and when a user is logged in.
You may get lucky with grep:
launchctl list | grep ssh-agent
In this case, it's the openssh prefernence that starts things:
launchctl list com.openssh.ssh-agent
I seriously prefer to edit these preference files with a tool like LaunchControl or Lingon. They give contextual help, graphical affordances and syntax checking. Apple's launchd/launchctl documentation exists but is dense, descriptive (rather than narrative / educational / prescriptive or basically easy to learn and use).
Here's an example how it shows an error with this specific agent on my Mac and a helpful warning that I need to disable SIP before futzing with this specific job or agent.
If you want to unload / disable this you need to find where the preference is stored then edit it or tell launchctl to unload (permanently) that item:
mdfind ssh-agent|grep plist
launchctl unload -w /System/Library/LaunchAgents/com.openssh.ssh-agent.plist
sudo launchctl disable system/com.openssh.ssh-agent
The daemons like this system one are being transitioned from the old "unload" syntax to the new "enable|disable" syntax, so you might get different results from my above commands on different versions and builds of macOS. Since you can see when it's running and kill it manually (
killall ssh-agent) you should have good success with this extra info on launch daemon that's not on most articles for unix management of this ssh tool.