Update: According to this forum thread, it seems that the set is indeed discontinued but will be re-released under a different number (10231 vs. 10213) with changes to improve stability. For those with the old set, you can call Shop@Home Customer Service and request a service pack for the stability upgrade.
Shop@Home status for 10213 Shuttle Adventure changed recently to “Sold Out” which typically means that the set is no longer in production and the supply chain is drying up. But, have no fear, Amazon is here. If you’ve been procrastinating on picking up quite possibly the best shuttle set ever made, now is the time to strike.
Thanks to pacific493 for the heads up.