Solution #2 (cont.)

This assumes you're using an open-source system with GNU Coreutils and the game is on a journaling filesystem (NTFS / Ext4 / APFS). It also assumes you're comfortable running scripts from strangers. Fortunately, I am no stranger to malicious software and wish not to cause anyone inconvenience, but still verify the script below will not cause your PC any catastrophic harm from a trusted, reputable computing expert before entrusting it and providing it agency to operate on your machine.

As I already explained what is necessary for the hotfix solution above, this script will also extract the texture pack and commit the fixes as described — except, with symbolic links. It's kind of long, sorry about that — it took me an hour to tinker with this and figure it out.

As for why to bother with this solution? Takes less space, and also preserves the original pack.
### This time, the script actually will work, guaranteed. RIp me one in PMs if it doesn't.
#!/bin/bash
### Debugging stuff; only enable when modifying.
# set -x
#trap read debug
#echo $PWD
# Resource pack path: Change to actual path if necessary
cd /mnt/Personal/minecraftMods/resPacks
# Set popd to pwd
pushd .
pack=Stay+True_1.16
sfx=-mod
# 1. Green birch leaves
mkdir -p $pack $pack$sfx/assets/minecraft
unzip -d ./$pack ./$pack.zip
cd $pack$sfx
# -> Pack attributes
ln -s ../$pack/pack.* .
cd ./assets/minecraft
# -> Duplicating pack structure for mod with links
ln -s ../../../$pack/assets/minecraft/* .
# !! rm is dangerous to use for directories! End-users should exercise caution when using such for this.
# -> Removing directories to modify in lieu of real directories; this will occur multiple times.
rm -rf ./optifine ./textures
mkdir ./optifine ./textures
cd ./textures
ln -s ../../../../$pack/assets/minecraft/textures/* .
unlink ./block
mkdir ./block
cd ./block
ln -s ../../../../../$pack/assets/minecraft/textures/block/* .
# -> Replacing birch textures
rm -rf ./birch_leaves*.png
# -> Copy used instead of links because Minecraft is picky
cp ./green_birch_leaves/*.png .
# -> Removing useless directory
rm -rf ./green_birch_leaves
# -> OptiFine textures
cd ../../optifine
ln -s ../../../../$pack/assets/minecraft/optifine/* .
unlink ./ctm
mkdir ./ctm
cd ./ctm
ln -s ../../../../../$pack/assets/minecraft/optifine/ctm/* .
unlink ./leaves
# -> This is all which is necessary for replacing all leaves
mkdir -p ./leaves/birch_leaves/orange
cd ./leaves/birch_leaves
# -> These are both the same.
ln -s ./orange ./cherry_blossom
cd ./orange
ln -s ../../../../../../../../$pack/assets/minecraft/optifine/ctm/leaves/birch_leaves/orange/* .
# -> Replacing with existing birch copies in texture
rm -rf ./*.png
ln -s ../../../../../textures/block/birch_leaves*.png .
# === END OF GREEN BIRCH LEAVES ===
# === START OF NO BUSHY LEAVES ===
popd
# -> Some unfinished business with Stay True: Invisible oak leaves fix
rm -rf ./$pack$sfx/assets/minecraft/textures/block/oak_leaves.png
# -> Variable for addon pack
nbl=ST_NoBushyLeaves
mkdir -p ./$nbl ./$nbl$sfx/assets/minecraft
unzip -d ./$nbl ./$nbl.zip
cd ./$nbl$sfx
ln -s ../$nbl/pack.* .
cd ./assets/minecraft
ln -s ../../../$nbl/assets/minecraft/* .
unlink ./textures
mkdir -p ./textures/block
cd ./textures/block
# -> Using copy from Stay True mod
ln -s ../../../../../$pack$sfx/assets/minecraft/textures/block/birch_leaves.png .Once this is done executing, pick the version with -mod (or whatever you defined for $sfx) at the end and use that instead.

What about for Windows?

Can't be arsed to replicate the above for Microsoft Windows, but if you want to make it easy on yourself and not stare at a command prompt, you can install Hermann Schinagl's Link Shell Extension utility and use that to much of the same effect (and much more quickly, too!)

Couldn't I just copy everything instead of this symlink nonsense?

That's not very I / O friendly for your solid state disk is it? Whatever, yes you can do that, and it would work just fine. I figured showing the symlink method would be more meaningful and maybe teach you something new, but if learning stuff like that isn't your style and you just want absolute minimum knowledge to get by, that's fine too.

Why not just make my own pack?

You can! See this.

Image credits

Thumbnail icons

Stay True logo: Trrigg
Chain: Mojang, Minecraft Wiki

Screencap

Stay True texture pack by Trrigg
VanillaTweaks