Key fields display in Lightning Experience as part of the Sales Path feature (not available in classic). To display Key Fields in Lightning Experience, we have to Enable the Sales Path. Lead and Opportunity object can have Key fields. You can include both Standard and Custom fields.
Navigate to: Setup | Customize | Path | Path Settings and click Enable.
Navigate to: Gear icon > Setup | User Interface | Path Settings and click Enable.
Note: Key Fields only reflect fields available in the Record Type selected for that Path.
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