Specifying specific shade values for elements no longer works

The bug

When models were first introduced, shade values could be specified as a number from 0.0 to 1.0 for each individual plane. From 14w25a onwards, this functionality appears to be broken, as it no longer works, and this persists to the latest version.

The 14w25a changelog did not mention that this feature was removed: https://web.archive.org/web/20140624075744/https://mojang.com/2014/06/minecraft-snapshot-14w25a-1-7-10-pre-release/

How to reproduce

Two resource packs are attached to this ticket, which replace the models for the first six leaves blocks. One works for the 1.8 snapshots, and the other is for the latest version.

1. Download both resource packs
2. Load up any version from 14w10c to 14w21b inclusive, apply the corresponding resource pack
3. Observe each type of leaves
4. Load up the latest version with the other resource pack
5. Observe each type of leaves

Expected results

In modern versions, each face would be shaded like was the case in the 1.8 snapshots.

Actual results

The leaves use the missing model, demonstrating that this functionality is broken.