Not sure if this is targeted or whether everyone gets this offer, but I discovered this by accident this evening.
If you are an existing Kindle Unlimited subscriber through Amazon and cancel your Kindle Unlimited subscription, Amazon will offer you to continue your subscription for $0.99 for the next 3 months, instead of the regular $13.99 per month ($41.97 for the 3 months).
I have only been subscribed to Kindle Unlimited for 5 months (first 3 months were free with purchase of the Kindle), but the majority of ebooks that my partner reads on the kindle are freebies, so we chose to unsubscribe. As soon as we did, we were offered the $0.99 deal.
Note that payments will restart at the regular $13.99 rate after the 3 months, so just remember to cancel before then.