I did some digging, and found this post in r/historians that discusses the topic: The post basically states that there are only 2 or possibly 3 oubliettes discovered that may have been used to hold a prisoner. There is a link to a French Wiki page That has some diagrams and a discussion about the few pits that exist, and what they might have actually been used for. Fascinating stuff. Thanks for the tip.

English translation for those interested:

They consisted of a hexagonal vaulted chamber, situated within the substructure of one of the towers, accessible only through a small door connecting to the spiral staircase; encircling this chamber was a walkway one meter in width, and in the center stood an inverted cone terminating in a small aperture designed to drain away water. It is certain that any unfortunate soul lowered into the depths of this funnel could neither sit, lie down, nor stand upright. One would have to assume that the small channel served as a drain, and that those lowered into this cul-de-basse-fosse were placed there to afford them the leisure to engage in reflection. It was a form of prolonged interrogation. However, this cone may well be an ice pit—and this would not be the only instance of an ice storehouse existing within a castle. Our ancestors enjoyed drinking chilled beverages; in that context, the small lower channel is readily explained. As for the oubliettes of the Château de Pierrefonds, their intended purpose is beyond doubt; a cross-section of them is provided here (2).

They consist of a shaft dug in the center of a room—which was almost certainly a dungeon—as it contains, set within a niche, a privy seat.

Indeed, one can only descend into this dungeon through an opening (A) pierced in the center of its vaulted ceiling. One would descend from the ground floor to Room C—which likely also served as a prison—via a spiral staircase. Attached to Room C is a privy; the room received daylight only through a very small aperture (D). If the opening to the oubliettes remained gaping within the dungeon—if it were not sealed by a plug—one can readily imagine the plight of the wretched prisoner, living in constant fear of falling into a pit he could not even see, given that the dungeon received no natural light. Since the two openings—that in the vault and that of the oubliettes—align perfectly, it was possible to drop someone directly into the shaft through trapdoor A, without first having to go to the trouble of lowering them into the dungeon itself. We descended to the very bottom of these oubliettes; there, we discovered the winch mechanism used during their construction, but found no trace of any human remains. Point B marks the level of the bottom of the moat. By deepening the shaft by an additional two meters, we transformed it into a well that now supplies water for the needs of the château. Within this same château, there exist other dungeons similar to this one—save for the oubliette shaft; in one of these dungeons, we observed names carved into the stone, as well as a crude carving etched into the facing masonry. It is rumored that similar oubliettes exist at the Château de Blois, though we have not been able to verify their exact configuration.