Yes, Nokia 8 does have a hybrid sim slot. You can use either 2 sim cards or 1 sim card & 1 MicroSD card.
Yes, Nokia 8 does have Always On display.
Yes, Nokia 8 is a true 4G network and it shall work with Airtel VoLTE.
Yes, Nokia 8 does have 3.5 mm headphone jack.
It’s safe to say that if a smartphone like this had shipped from the Nokia stable a few years ago, the Finnish company might never have had to step back from the mobile business.