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Posted a reply to WordPress Feed Issue – ‘Use of unknown namespace: com-wordpress:feed-additions:1, on the site WordPress.org Forums:
It's probably JetPack. See jetpack/modules/enhanced-distribution.php add_action( 'rss_head', 'jetpack_enhanced_distribution_feed_id' ); add_action( 'rss_item', 'jetpack_enhanced_distribution_post_id' ); add_action( 'rss2_head',…