Not actually a bug, but reporting anyway.

Forge has deprecated Minecraft's IBakedModel in favour of net.minecraftforge.client.model.IFlexibleBakedModel (used in the Turtle upgrades). Not sure though if there are issues with rendering a IFlexibleBakedModel compared with the original one.

For getItemModel you can always wrap the IBakedModel with IFlexibleBakedModel.Wrapper.

I presume the issue is that ITurtleUpgrade.getModel returns IBakedModel, and this is triggering a warning when you compile your project? IFlexibleBakedModel inherits from IBakedModel, and the code that calls it is cable of handling either. The reason the return value is IBakedModels is to allow the user to implement either one, as most built-in models will simply be IBakedModel.

I guess the deprecation warning is more for implemeters rather consumers. To me it feels a bit odd using a deprecated interface in the API but it isn't a big issue.

Toyed with changing this, but decided to keep it how it is, using ISmartBlockModel/ISmartItemModel as precedent. Closing.