Didn't see anything posted about this and just remembered myself.
If you redeemed Microsoft Reward points for a gift card, odds are there's a note that says this with it:
PROMOTIONAL GIFT CARD. REDEEM BY JUNE 30, 2018 AND SPEND WITHIN 90 DAYS OF REDEMPTION.
Go to https://www.microsoft.com/redeem
to redeem your cards. Be sure to spend them within 90 days.