In the objects view we have this option called "Unreferenced Objects" which will show unused objects of the types mentioned below:
Is there a way to get a list of the unreferenced process models in the application so that they can be removed from the application?
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I do not know of a way to include process models here.
Just as some background information I think it would not make sense to include process models into this filter since it does not mean that a process model object is not being used. Think of process models being initiated by triggers or being configured as application action.
Also I do not see any practical way other than checking all process model dependents one-by-one. Keep in mind that unless you have 1000+ process models this can be done in little time still.
Also something you can do is explain your need to Appian and create a feature request for this via MyAppian.