I’m just having a browse around to see if we can glean anything about paṇḍaka. One of the few non-Vinaya texts we find it in is the Milinda:
The translation is less than ideal:
The Milinda is of course a late text, and this is one of the later chapters. Not one of our finest moments, it says you can’t trust a woman with secrets because of her weak and changeable understanding (itthī paññāya ittaratāya) thus endorsing the position that the suttas ascribe to Māra.
Of paṇḍakas, it says they can’t be trusted with secrets due to their “ambiguity” (paṇḍako anekaṃsikatāya, not “imperfection” as the translation has it.) Ekaṁsa literally means “one-sided”, and is used for example for philosophical questions that can be answered definitively as opposed to those that cannot (anekaṁsa).
Whether this is directly relevant in the context of the Vinaya is an open question, but so far as it goes, it confirms that the issue with paṇḍakas is not that they have been something different in the past, but that in the present they are ambiguous. Of course, the question as to why ambiguity is considered a problem is really the interesting thing here, but for now I’m just checking what the texts say.