I don’t think there’s currently a ‘backlog’ of planes.
Most more complex planes require WASM to be implemented for XBox properly. That is scheduled to come out with Sim Update 11. So until that time, whenever third parties release aircraft that don’t require WASM will just be added when they’re released (generally on the thursday marketplace update).
Once SU11 is released, it will likely take those developers some time to implement it, and release.
Other than the SU11 WASM release for XBox, there’s really nothing MS/Asobo can do / know about third party releases.