The pack isn't going to expire (and neither has any of the other packs. They're all still available from the XBL store page, just not the in-game store.), but IIRC, they're currently not in the REQ pool. 150k RQP or the paid option atm.
Oh, on the community pop-up for Halo 5 they say that the pack is a limited time only. That's what I meant by expiring. I didn't know the packs are still available on the Xbox Store.
Anyways I'm on the fence about getting the pack or just waiting for the possibility of the helmets being added to the req pool (which I'm pretty sure they'll do, right? RIGHT?!)
Also Halo Wars 2 ads: