Contrary to what some others are saying...

I don't really think Denji's character really "regressed" the way they claim he was. At least, I wouldn't say that Denji is purely simping for Asa/Yoru by complying with the "arm eating" request, nor is he staying dumb / ignorant. If anything, from what his expressions and dialogue seem to suggest, this is the most alert he's ever been, and the most he's ever wanted to be informed and look for answers.

(Picture 1) Denji checks with Yoshida on what exactly happened during the time he was unconscious and Pochita was rampaging, up to and including what Asa did. The old him in the first part, and the earlier bits of this part (like in his first table conversation with Yoshida) never really seemed to put his back into wanting to know, much less care, about the current situation at hand.

(Pictures 2 and 3) Denji comments that he has already been suspicious of Asa being more than what she seems to be, even mentioning her regrown arms. When Yoru exposes herself as a split personality of Asa's, Denji's reaction, while surprised, is less completely blindsided, and more "for real" (as if he already anticipated it as a major possibility). It more or less confirms that Denji did properly take note of Yoru's earlier actions (her mentions of the "other (her)" and her healed arms) more than he initially seemed.

(Picture 4) While the action in itself could be misinterpreted as regression, it goes mostly unnoticed that unlike past cases, where Denji accompanies his fawning with a blank or dumb look, he still keeps a focused, possibly serious and disgruntled even, look to Yoru's request. It feels less of blind simping like before, and more that Denji genuinely really wants to get more of the information he needs and has to know, of which personality can be trusted as a companion. And given Asa / Yoru's singsong nature, who'd blame him?

(Picture 5) Even as Denji follows Yoru's deal to keep his arm in his mouth, he still keeps a (mostly) attentive gaze at the ensuing conversation, especially to important parts such as Yoshida's question to Yoru about why Denji hasn't become her weapon yet. He is still listening to any other points the others have to say, taking in all that he finds necessary.

(Pictures 6 to 8) Continuing on the point from picture 5, Denji not only immediately reacts to Yoshida's request to Yoru to turn Denji into a weapon immediately, but also confirms from just his subtle comment about averting the coming of Death Devil, that Public Safety had intentionally planned for Nayuta to die and him to release Pochita. The only clues he had before this was Yoshida's comment at their first meeting, and Barem's comment at the sushi restaurant.

To conclude, while you can still say that Denji's craving for worldly pleasures is still an encompassing problem, to say that he "regressed" by being mindless and ignorant, is a bit of a skewed conclusion to make, when this is probably him at his most alert, deductive, and most willing to keep informed about everything. I genuinely don't really know how people conclude the former from this.