Please note that a resource pack is used in the attached screenshot because this issue is about the resource pack mechanics. The default textures cannot be used because they do not utilize the feature in discussion. I fully understand if this feature was accidental and is unsupported, and thus this report is invalid, but I wanted to report this nonetheless in the case that the feature is supported.

In Minecraft 1.8, when the fast graphics leaf textures were removed, the game instead just read the alpha as black when fast graphics mode was enabled, and removed the ability to customize the fast graphics background. However, I discovered that it was still possible to customize the background by using opacity.

That is to say, you could make the fast graphics texture, drop the opacity to 1% so that it is essentially invisible, and then overlay the fancy graphics version on top of it. When fancy graphics was enabled, the game would regard the low opacity to be invisible, but when fast graphics was enabled it would make it fully opaque, essentially having the same effect of the pre-1.8 textures.

This is still the case, but as of 1.15, using this method causes the leaf item models to appear fully opaque, while the blocks remain transparent. The attached image gives an example of what I'm talking about.

I realize that this method likely wasn't intended to work in the first place, but it did allow for a lot more customization, and enabled fast graphics to be a lot more aesthetically pleasing. And given that it was fully functional until relatively recently, I thought I'd report it anyway on the off chance that the previous functionality could be added back.
 

Resource Pack Link: https://www.curseforge.com/minecraft/texture-packs/immersion/files/3179317