Verse always tends to be linguistically conservative in language (archaic words). The Sutta-nipāta, Dhammapada (both edited in Khuddaka-Nikāya), and Sagātha-vagga (in Samyutta-Nikāya/Samyukta-āgama) were composed in verse. These texts in verse were considered by some scholars an earlier compilation, which is mainly based on their relatively archaic language.
But this reasoning is unsound, because, as stated above, verse always tends to be linguistically conservative in language. This relatively archaic language is not regarded as the formation of early Buddhist texts, according to Ven. Yinshun.
See
The Fundamental Teachings of Early Buddhism pp. 7-11. The author, Choong Mun-keat, indicates that Ven. Yinshun has demonstrated the historical important of Samyutta-Nikāya/Samyukta-āgama in Early Buddhism in two books:
The Formation of Early Buddhist Texts 原始佛教聖典之集成 (1971), and
Combined Edition of Sūtra and Śāstra of Saṃyukta-āgama 雜阿含經論會編 (1983).
See also the same author provides further useful information on this topic/issue in the following recent paper:
“Ācāriya Buddhaghosa and Master Yinshun 印順 on the Three-aṅga Structure of Early Buddhist Texts” in
Research on the Saṃyukta-āgama (Dharma Drum Institute of Liberal Arts, Research Series 8; edited by Dhammadinnā), Taiwan: Dharma Drum Corporation, August 2020, pp. 883-932.