How do you identify orphaned classes?

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Been cleaning up our production 2017.1 instance of Health Connect in preparation to a move to Health Connect 2020.1 (on IRIS!)

One thing I have found I we have built up a number of old classes and other configuration files.

It would be nice to start on the new server with a clean slate.

Is there any way to identify classes that haven't been used for a long time so they can be reviewed and removed?

Kind regards, 

Stephen