In our V4 SDK the legacy APIs were deprecated in favour of our C-style LeapC API which is closer to the metal, the documentation for LeapC can be found here. As part of the V4 release we also shared our wrapper for the legacy language bindings, which makes it easy to integrate legacy-reliant applications with LeapC.
You can find the binding tool here: https://github.com/leapmotion/LeapCxx
Alternatively, these legacy APIs are still included (although deprecated) in our older V3 Orion release which can be downloaded from our developer site here with documentation available here.