The email does say "
Valid for travel booked and flown 1/1 - 5/20/21" right in the announcement. That said, there are a few interesting bits in the fine print; emphases in bold below are mine. It excludes a lot of purchase methods, but not Travel Bank, from what I can see. I am guessing the last bit regarding rebooking suggests that you can call and rebook existing flights under the current free change policy; not sure if they'll honor the original fare or not. Seems like it basically gets processed as 50% off for each passenger on the back end.
On most airlines and in most years I'd be irked about the 50% points, but with the qualification requirements lowered and given the availability of points pooling, it's not so bad.
Must be exactly two (2) travelers in the booking and include the Mosaic member and one (1) other travel companion, neither of whom can split themselves off the flight booking. Valid on JetBlue-operated flights booked and flown 1/1/21 - 5/20/21, with all travel completed by 5/20/21. Offer does not apply to non-revenue or travel booked with a travel certificate, award or redemption flights, flights booked in whole or in in part with points, Cash + Points redemptions bookings, etc.; is not valid in connection with partner or codeshare or interline flights, cruises, JetBlue Vacations packages, or any other product or service; may not be combined with other special offers or discounted travel; is not redeemable for cash; and has no cash value. Both the Mosaic member and their travel companion must be booked by JetBlue at the same time, in the same class of service, in the same record, and must both be present for check-in and travel all segments of their itinerary together. JetBlue may suspend a Mosaic member's companion benefits at any time for abuse of this program, including for fraudulent behavior. Requires payment of applicable taxes and fees. Offer is for new purchases only and cannot be applied to previously issued tickets. Offer equates to 50% off the base fare of each of the two (2) tickets purchased (for the eligible Mosaic member and their travel companion). Both travelers are eligible to accrue TrueBlue points according to the fare paid for each ticket at the 50% discounted rate only. Subject to availability, if both booking date and travel date are within the terms of the offer, JetBlue can cancel and rebook a new reservation with this same offer. Offer is non-transferable. Other restrictions apply.