As I discuss in this answer, each of the four Vedas consists of four portions: Samhitas, the core part of the Vedas which consist of mantras heard from the gods; Brahmanas, commentaries on the Samhitas which provide instructions on the proper conducting of important Vedic Yagnas; Aranyakas, which provide instructions for Yagnas meant for forest-dwellers and hermits; and Upanishads, which consist of conversations between teachers and students which clarify the philosophical teachings of the Vedas. But as I discuss in this question, the Isha Upanishad has a very different origin: it isn't some supplement or commentary, rather it is part of the Samhita of the Shukla Yajur Veda.
Now as I mentioned above, the mantras of the Samhitas were generally used in Vedic Yagnas (fire-rituals), in a precise fashion delineated in the Brahmanas and Aranyakas of the Vedas. But in this excerpt from his commentary on the Isha Upanishad, Adi Shankaracharya says that the mantras found in the Isha Upanishad were never used in Vedic Yagnas:
Adoration to the Brahman. The mantras beginning with Isavasyam, etc., have not been utilized in rituals, because the serve the purpose of enlightening us on the true nature of the Atman, who is not an anga of, i.e. not connected with, Karma. The true nature of the Atman consists, as will be described, in its purity being untouched by sin, oneness, being eternal, having no body, omnipresence, etc., and as that conflicts with Karma, it is only reasonable that these mantras should not be utilized in rituals[.]
I discuss the issue of knowledge of the Atma not being a supplement to Karma here. But my question is, is Adi Shankaracharya correct that the mantras of the Isha Upanishad were never used in Vedic Yagnas?
Do other commentators on the Isha Upanishad agree or disagree with Adi Shankaracharya here? If it helps, here is Madhvacharya's Dvaita commentary and here is Vedanta Desikan's Visistadvaita commentary; neither of them seem to address this issue.
Also, do any Brahmanas or Aranyakas of the Vedas reference the Isha Upanishad in their descriptions of Yagnas? The Brahmana of the Shukla Yajur Veda is the Shatapatha Brahmana, and as I discuss here it contains an Aranyaka within it, but often Brahmanas and Aranyakas from one Vedas will reference mantras found in another Vedas. So it's possible other Brahmanas and Aranyakas may reference it. Also, does anyone know if the Isha Upanishad is mentioned in Shrauta Sutras, since they compile instructions for various Vedic Yagnas?
