Both processes referenced in this list are INACTIVE. Can they be just deleted?

One of these may reference the field, but so far I haven't found one of them doing that.

Below is a sample of the Classic Email Template that is used with Email Alerts in Workflows (and processes).

I don't see any reference to the offending field on this email template or in the email alert action that goes with it.

I unchecked the required checkbox. This says that in order to save a campaign record, this field must have a value...i.e. one of these have to be chosen in order to save the record. This may solve the problem.