Even if Nanachi died here, this series has already demonstrated ways to cheat death. Riko herself was a stillborn baby and somehow revived through the use of a relic. Then there was Mitty who was supposed to have been physically deleted by Reg and somehow another copy of her still existed somewhere after they dramatically sent her off. Srajo I'm sure also did not talk about the nature of souls for nothing. Maybe a Chekhov's gun waiting to be fired.
The point is, if the author wanted to, he has already established ways to bring her back, assuming she IS dead here and not just critically injured, basically waiting for an "upgrade" to give her more useful abilities via relics etc. since narratively speaking, her "knowledge of the abyss" stopped being useful once they left layer 6 or earlier. Arguably, as a character she was most important in layer 4, but that was ages ago.
Side characters have been killed off before, but a main character? I don't remember. Even Bondrewd somehow survived, yo. I'm being skeptical. I'll believe Nanachi's permanent death when the characters themselves confirm it.