It is the app formally named "workflow" that apple acquired some time ago. It got renamed to "shortcut" for iOS 12 and has this ssh action in it.
If you already had the old "workflow" app it will be updated to the new "shortcut" app, if you do not have it you need to install it from the App Store.
I am familiar with shortcuts, but I didn't think that there was an actual SSH action that was shipped with it, I thought there was an SSH action that an application may be providing.
I'll have to check it out.
In iOS settings > workflow (yes, it is still labeled workflow in iOS settings!), it credits NMSSH which appears to be an objc wrapper on top of libssh2.
No idea, I didn't dig into it. I think it had the general "Settings" gear as the icon so maybe it's provided by iOS?