This information is quite hard to obtain from the system. You will need to use SAS MA Integration Utilities to export campaign objects into XML and obtain that information from there. Note that extracting not just one, but all campaigns may not be an easy task and will require fair degree of automation efforts.
Most often, you can substitute the original question "which campaign diagrams still exist in the repository are using the data item" with a very close, but a bit different question "which campaigns that were actually executed (or counts updated) in last XX days were using the column during that execution".
This information can be obtained from the Customer Intelligence Core log, searching by the physical column name in SQL queries.
Hope this helps.
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Dmitriy Alergant, Tier One Analytics