WikiExporter has an explicit check that makes it fail if the SCHEMA_COMPAT_WRITE_OLD flag is not set in $wgMultiContentRevisionMigrationStage. This causes all exports to fail in SCHEMA_COMPAT_NEW mode.
This check was needed to avoid breakage while WikiExporter was still relying on the old schema for bulk operations. This has has however been however been resolved by T198706: Make BackupDumper, WikiExporter and XmlDumpWriter compliant with the MCR revision retrival mechanism (main slot only) and T198341: Remove all references to the rev_text_id and ar_text_id fields. This check is now obsolete and should be removed to allow us to switch to SCHEMA_COMPAT_NEW everywhere.