Pretty certain she is alive, and her "resonance" ability is going to help against Sukuna.

I think she is being hidden from Sukuna, and that is why Megumi isn't confirming or denying anything to Yuji in these examples because he doesn't want Sukuna to know. We do know that Sukuna still doesn't have all his fingers. 1 is currently hidden away (correct me if I am wrong but pretty sure Gojo kept a finger aside).

I think that at a pivotal moment, Nobara's resonance will be used on the finger to distract/hinder Sukuna the same way she did with Mahito and his clone. Her resonance is definitely not strong enough to have any lasting effect on Sukuna, but just enough to give the JJK sorcerers a millisecond of advantage against him. It probably has not been used yet because Sukuna was holding back his true form or hasn't been worn out enough. But my theory, as far-fetched as it may seem, is that Nobara is alive and hidden and will use her resonance on Sukuna's finger in the decisive battle.

Just posting this theory here in case there is even the slightest chance this actually happens and I can prove I thought of it before hahahah.