https://www.reddit.com/r/DestinyTheGame/comments/7i1l2n/you_can_get_sonyms_to_refund_your_purchase_thanks/?st=JAVVKCIW&sh=f370bbfd
This Reddit post pretty much covers everything. Locking the platinum achievement behind a paywall was against the rules of the initial purchase agreement, GO AND GET YOUR REFUND!
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#destiny2
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Good luck getting a refund based exclusively off the fact that you can't unlock all achievements.