I just saw bitrefill doing 50% cashback for NordVPN gift cards. Or is there a catch to the said cashback? I couldn't find anything from self-help articles and ToS, though.

The cashback can only be used to buy off of bitrefill ofc, so, promo takers must have something in mind to buy. I think this gift card is for the basic plan but with the exception of 6 devices instead of 10. For the 1 year plan, it's slightly cheaper at bitrefill but Nord offers an additional 3 months on their website.

Source: https://bitrefill.com/us/en/gift-cards/nordvpn-international/