Deleting fields used by cross-object formulas
Key Takeaway
Salesforce won't let you delete a custom field while any formula still references it. Before you delete the field, find every formula that uses it and remove the field from those formulas.
A cross-object formula is a formula on one object that reads a field from a related object. For example, a formula field on Case can show a value from the related Account. This matters when you delete a field, because a formula on another object can block the delete.
How a cross-object formula spans related objects
A cross-object formula spans two related objects and references merge fields on those objects. It works through lookup relationships and through master-detail relationships, where the detail side can reference fields on the master (parent) object.