There was a technique to achieve transparent textures on meshes by using a Decal on a SpecialMesh, as explained in this thread: How to make a Mesh texture actually have transparent parts
Quote from @zeuxcg (Roblox Staff) in that thread:
By “this technique”, you mean using decals on special meshes, right? We’re unlikely to ever stop supporting this. Do note that this is not efficient rendering wise - we’re essentially rendering these objects twice, second time around with more overhead to be able to combine texture with a previously rendered part’s color. I go into this a bit in my RDC presentation from this year.
But this method of applying decals to SpecialMesh has recently stopped working, instead it applies Decals as it is applied to Parts, just on the Face specified on the property of the Decal. Leaving developers with no options to achieve transparent textures (and also breaking existing implementations).
This butterfly used to have proper wing placements and a face texture when Decals was UV-mapped, after the change in behavior of Decals it looks like this: