As its mentioned here that Standard blob metadata properties can be extracted into the fields as below: The blob indexer automatically creates internal field mappings for these blob metadata properties. You still have to add the fields you want to use the index definition, but you can omit creating field mappings in the indexer.
Use metadata_storage_last_modified (Edm.DateTimeOffset) and make it filterable and facetable: last modified timestamp for the blob. Azure Cognitive Search uses this timestamp to identify changed blobs, to avoid reindexing everything after the initial indexing.
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