MacRumors got in touch with an Apple support rep who confirmed that the devices are no longer available from the stores, but the base stations will still be available online. In meantime if online ordering isn’t your thing, the smaller AirPort Express can be found in the stores, so you can opt for that if that works.
So why did Apple remove the devices? The company has yet to provide a reason, but MacRumors speculates that it could be due to an FCC deadline regarding router software security. “Starting June 2, 2016, permissive changes will not be permitted for devices approved under the old rules, unless they meet the requirements of the new rules. All devices partially or completely approved under the old rules cannot be marketed starting June 2, 2016 unless they meet the requirements of the new rules in all the bands of operation.”
We’re not sure what this means for the devices and their future, but they have been proven to be rather useful in the past, so hopefully Apple will release newer models that comply with the FCC’s new guidelines soon.