I don't know how long the sale is going to last and I don't know of a way to look at the underlying fare or fare rules. Norse Atlantic doesn't show up on ITA Matrix and it doesn't show up in ExpertFlyer. It's not like an official sale that Norse Atlantic has announced. Not every route is cheaper (some have actually increased in price) and premium economy is definitely more expensive than it used to be!

I will say that I started tracking N0 flights in September and this is the cheapest economy has been on this route - Norse Atlantic - JFK to LGW Fares - 20240129 | Tableau Public (uploaded a portion of my data to Tableau Public to easily host the data)

Anyways, Norse dropped prices on a few more routes today.

Earlier MIA-CDG was $129 one-way, but it is now $160 one-way.

MIA-LGW - $129 one-way - https://www.google.com/travel/flight...TKV3ygbMaXA1z8
MCO-LGW - $129 one-way - https://www.google.com/travel/flight...rSSiQVmwUa6X99
LAX-LGW is still $196 one-way - https://www.google.com/travel/flight...JV6x5hhTixpNR6

EDIT - You can still fly CDG-MIA for $129 one-way - https://www.google.com/travel/flight...UDEXQCUysz3N19 or OSL-MIA for $129 one-way - https://www.google.com/travel/flight...8bnZ6br9AbTeP8