I am still on 7.3 so I don’t know. I’m just telling you what your error message means.
According to this thread no known documentation exists for PluginHelper in 7.0:
https://community.sailpoint.com/t5/IdentityIQ-Forum/PluginHelper-methods-documentation/td-p/12357
If 8.x has better documentation, you should refer to that. Or try reading the JavaScript source code as suggested by the thread.