If you've connected a platform to Funnel which excludes certain fields, you will no longer see them listed in the Data Explorer or when creating custom fields.
You will however see the entire list of fields that Funnel can pull from these platforms in the Dimensions & Metrics sections:
We hope that this update makes it easier to find and select the fields you need. If you have any questions please contact support@funnel.io and we'll be happy to help!