Regarding the subject of whether there are interpolations in the scriptures or not, there are currently two perspectives:
Those who declare every portion of the scriptures that contradicts their own understanding as interpolated.
Those who hold the belief that no part of the scriptures is interpolated.
Individuals of the first type are dismissible. Their claims of interpolation are not based on scriptural analysis or the tradition of the texts but rather on the sentiments of the general public or their own feelings. They label those portions of the scriptures as interpolated which their own intellect deems incapable of understanding, even if these portions are accepted as authentic within scholarly tradition and are described in multiple scriptures in various contexts and rules.
Individuals of the second type, although they are highly scriptural in their approach, cannot entirely rule out the possibility of interpolations in the scriptures. While scriptures that have been preserved through continuous study and commentary may be less likely to have interpolations, those that have not been widely preserved in manuscripts or through study traditions are fully susceptible to interpolation. We observe such occurrences even before our eyes even today. To clarify this point, the Markandeya Smriti says:-
किञ्च वेदाश्च सूत्राणि कल्पाः शाखाः समन्ततः ॥ अध्यापनाऽध्ययनतः
पदक्रममुखेन वै। अतिस्पष्टाः सुप्रसिद्धाः प्राजापत्यादिसंज्ञया ॥
इत्यादि तस्मादेतेषु सर्वेषु विद्याभेदेषु कृत्स्नशः । उपदिष्टेषु विशयो
नास्त्येव सुतरां खलु ॥
नैवं स्मृतिषु सर्वासु पुराणेष्वखिलेषु वा । तस्मात् तन्मध्यसम्प्राप्तप्रक्षिप्तग्रन्थविस्तरैः ॥ तानि
सर्वाण्यप्रामाण्यकलङ्कितशरीरतः । आचारसंशयेष्वत्र सम्यक् स्युर् न
विधायकाः ॥
(मार्कण्डेयस्मृति)
The above statements clarify that the Vedas and Vedangas, being transmitted through traditional means, are extremely clear. However, the same level of clarity does not apply to the Puranas, Smritis texts, which have undergone many interpolations and have many portions tainted with doubt regarding their authenticity.
Even Scholarly Acharyas have also spoken about interpolations in Smriti/Purana texts from quite the early times.
पुराणवचनेषु पद्मपुराणादिकतिपयपुराणेषु कृत्रिमवचनानि बहूनि दृश्यन्ते।
In the Puranas, such as the Padma Purana and other numerous Puranas,
many interpolated statements are to be seen.
[Bhattalaxmanacharya's Tithiphalaadhyaya (13th century), 27.43]
In the Nirnayasindhu's पाणिहोमप्रकरण, Bhattakamalakar has also stated that certain verses of the Brihannaradiya Purana are to be disregarded because they are not honored by the elderly. (यत्तु बृहन्नारदीये 'अनग्निर्दूरभार्यश्च... इति' तद् वृद्धाऽनादरादुपेक्ष्यम्।)
"In such a situation, the question arises: what is the decisive criterion for determining interpolations? If we say that being contrary to the Vedas is a characteristic of being interpolated, this is not accurate. This is because in the six philosophical systems (Shad-darshanas), there are numerous contradictory principles concerning the Vedas, such as nitya versus anitya, impersonal versus personal, the theory of prakriti - parinamavada vs the theory of Brahma-parinamavada, and most importantly the question of bheda or abheda between jiva and brahman. In this context, the six philosophical systems are not proven to be interpolations; rather, their validity is established based on different processes.
Also saying 'This portion is interpolated because such and such a character cannot do this' is also fallacious. In that case, one might raise objections such as like 'No father without fault can give his son a harsh 14-year exile' or 'A righteous person like Yudhishthira cannot gamble away his wife'...that would imply that the entire Ramayana and Mahabharata should be considered fictional.
Therefore, ultimately, the criterion of interpolations should be through - "“शास्त्रपरम्परानैरन्तर्यविच्छिन्नत्वे सति स्वसमानकोटिशास्त्रविरुद्धत्वे
सति शिष्टपरम्परया अपरिगृहीतत्वम्” (being inconsistent with the continuity of the scriptural tradition and contradictory to texts of the same category, and not accepted by the traditional scholarly lineage).