Hello Markus,
you might try the following:
go to the Report | Report Template List and select the "Interface Connectivity Summary Report". There you select a time frame (i.e. 90 days). In the report you will find the coloum "Total Break Persistance", which will tell you the minutes the interface was down. If this value is 60 x 24 x 90 = 86400 Minutes you know that this interface is not in use for at least 90 days.
Keep in mind to set the configuration poll intervall to an appropriate value.
Best Regards,
Michael