It would be really cool to see if these buoys could make a return, but it seems unlikely, as @Jip mentioned.
In hindsight, however...
I don't think it'll actually be as useful as having the sonar on the unit itself, as the plane can move much faster and therefore be able to cover a larger area quickly.
That is, unless the Spy plane is dropping a buoy that covers the size of a 10 x 10 km area or even just multiple buoys.
Nevertheless, if this ability is ever brought back to life somehow - obviously with some tweaking and all - I'm sure it'll make a great addition to FAF.
~ Stryker