External objects are excluded from data backups, as of 22nd of November 2019. These objects have a __x suffix, and the associated records will appear as deleted if they have previously been backed up.

The reason for excluding these object types is that the data resides outside of the Salesforce org, and therefore should be backed up directly from that source (rather than via the Salesforce org that the data has been made available in). As some of the consistency and conventions that Salesforce enforces for "native" data can be circumvented for external objects, they cannot be relied upon when storing this data, causing data integrity issues for backups.

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