The scriptures of Buddhists and Jains are composed in overwhelmingly incorrect (asadhu) language, words of the Magadha or Dakshinatya languages, or even their dialects (tadopabhramsa). Therefore false compositions (asannibandhana), they cannot possibly be true knowledge (shastra) … By contrast, the very form itself (the well-assembled language) of the Veda proves its authority to be independent and absolute.
-also sayeth Kumārila Bhaṭṭa.
Tibetan historian, Buton, 14th century: the Sthaviravadins use Paisaci (which he does not call Prakrit).
Rajasekhara (approx 900CE) in Kavyamimamsa:
The people of Avanti, of Pariyatra, and of Dasapura [Chattisgarh] use Bhutabhasha [Paishachi].
There is a chapter dedicated to Paisaci in Vararuci’s Prākrita Prakāśa, and in another work by Hemacandra. I wonder if the description given therein matches Pali.
If it does, these eminents had considered Pali a Prakrit, albeit an archaic one.
If it doesn’t, the great historian Buton was perhaps mistaken in his identification.
(Edit: I checked, nothing specifically like Pali in the 14 Paisaci aphorisms. But given that this text considers Sauraseni as the base language, for which many more aphorisms are given, there is still overall similarity. Maybe Buton was using the term Paisaci as a geographical designation for the language of the Deccan, or maybe in the sense of admixed or “low” vernacular).
You could see Pali as MIA, or you could see Pali as Old Prakrit. The reason why Kumārila Bhaṭṭa doesn’t want to see Pali as Prakrit is that he doesn’t want to give it the status of a regular grammatical language, because his whole argument against Buddhism is based on the linguistic inferiority of Buddhist texts.
One legend was that Kumārila Bhaṭṭa’s nephew, Dharmakirti, went on to become a great Buddhist philosopher just to refute him. If Kumārila Bhaṭṭa was alive today, he would probably be a VHS or RSS (these are Hindu supremacist groups) member. He was a fundamentalist, not a linguist.