As the title states, there are no explicit references to Nirodha Samapatti in what is widely considered to be the earliest stratum of the Pali canon (which includes texts such as the Sutta Nipata, Atthakavagga, Parayanavagga, Itivuttaka, Udana). I'm interested to hear from those who consider Nirodha Samapatti to be a canonical teaching of the Buddha - why do you think there are no references to this attainment in the aforementioned texts?
There is no evidence the Sutta Nipata, Atthakavagga, Parayanavagga, Itivuttaka, Udana are earlier texts.
The Itivuttaka, Udana & many Sutta Nipata suttas (such as the Metta Sutta and Maha-Mangala Sutta) appear to be later texts.
The Atthakavagga & Parayanavagga of the Sutta Nipata were spoken to disciples of a Brahmin and contain very general simple teachings. Therefore, there was no need to mention nirodhi samapatti to these Brahmin students.
The older language used in the Sutta Nipata is similar to deciding to write poetry in Old English. The older language of the Atthakavagga & Parayanavagga is suitable for writing in verse (poetic style). Sujato has explained this many times.