Over the course of the last few weeks I have spent my money testing whether or not this issue would go away but even the event packs have given me legendary arena xp boosters.
If you’re getting legendary XP boosts from event packs then it sounds like you might be done with the REQ system now. You can test this by opening a Gold pack - if you get 2 x 3000RP cards refunded back, then you have finally completed the Halo 5 REQ system. Some items might still need to be unlocked from Bronze packs, some require commendation completion and a few require the purchase of a pack with real money