ऋषभोपानहोर्ञ्यः

Operational rule vidhi

The affix ञ्य (+/- य) comes, in the sense of a primitive serviceable for a product after the words ऋषभ and उपानह्, denoting the products.

Word by word (padaccheda)

Target ऋषभ- उपानहोः ṣaṣṭhī · dvivacanaSubstitute / term ञ्यः prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

ऋषभ-उपानहोः ञ्यः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

ऋषभ- उपानहोः → ञ्यः

Replace ṛṣabha-upānahoḥ with ñyaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The affix ञ्य (+/- य) comes, in the sense of a primitive serviceable for a product after the words ऋषभ and उपानह्, denoting the products.
Thus आर्षभ्यो वत्सः, औपानह्योः मुञ्जः ॥ This 'nya' debars 'chha'. Even when the primitive is 'leather चर्म', the affix 'nya' will come after उपानह् irregularly debarring अञ् of 5.1.15. Thus औपानह्यं चर्म ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.