Edit: this mod is no longer necessary as of v1.9 of https://www.curseforge.com/minecraft/mc-mods/biomes-youll-go-1-12-2-fixed

Passable Leaves + BYG Compatibility (1.12.2)

Makes BYG's leaves behave like normal Minecraft leaves under the Passable Leaves mod. You can walk through them, get slowed down slightly, and hear rustling sounds.

Requirements

• Passable Leaves 2.0.0+  
• Oh The Biomes You’ll Go (BYG) 1.8.0.1  (I recommend this fork: https://www.curseforge.com/minecraft/mc-mods/biomes-youll-go-1-12-2-fixed)
• MixinBooter 10.6+

How It Works

This mod adds a Mixin that adjusts BYG’s MCreator leaf blocks to match real leaves:

• Removes their solid collision box  
• Sets them to render in the correct leaf layer  
• Lets Passable Leaves’ slowdown/sound effects apply normally

Credits

Compatibility mixin and patch by Akiak.   Passable Leaves by Team RTG.   BYG by AOCAWOL.

To Make BYG Compatible With BetterFoliage/RLFoliage:

Add the following classes to betterfoliage.cfg under leavesClassesWhitelist:

        net.potionstudios.byg.Aspen_leaves$BlockCustom
        net.potionstudios.byg.Baobableaves$BlockCustom
        net.potionstudios.byg.Birchleavesyellow$BlockCustom
        net.potionstudios.byg.Brownbirchleaves$BlockCustom
        net.potionstudios.byg.Cherryleavespink$BlockCustom
        net.potionstudios.byg.Cherryleaveswhite$BlockCustom
        net.potionstudios.byg.Cikaleaves$BlockCustom
        net.potionstudios.byg.Cypressleaves$BlockCustom
        net.potionstudios.byg.Ebonyleaves$BlockCustom
        net.potionstudios.byg.Enchantedleavesblue$BlockCustom
        net.potionstudios.byg.Enchantedleavespink$BlockCustom
        net.potionstudios.byg.Enchantedleavespurple$BlockCustom
        net.potionstudios.byg.Firleaves$BlockCustom
        net.potionstudios.byg.Frozenoakleaves$BlockCustom
        net.potionstudios.byg.Greatoakleaves$BlockCustom
        net.potionstudios.byg.Hawthornberryleaves$BlockCustom
        net.potionstudios.byg.Hawthornleaves$BlockCustom
        net.potionstudios.byg.Hawthorn_leaves_flowering$BlockCustom
        net.potionstudios.byg.Hollyberryleaves$BlockCustom
        net.potionstudios.byg.Hollyleaves$BlockCustom
        net.potionstudios.byg.Ironwoodleaves$BlockCustom
        net.potionstudios.byg.Jacarandaleaves$BlockCustom
        net.potionstudios.byg.Mahogany_leaves$BlockCustom
        net.potionstudios.byg.Mangroveleaves$BlockCustom
        net.potionstudios.byg.Mapleleavesred$BlockCustom
        net.potionstudios.byg.Mapleleavessilver$BlockCustom
        net.potionstudios.byg.Oakleavesdrybrown$BlockCustom
        net.potionstudios.byg.Oakleavesdrygreen$BlockCustom
        net.potionstudios.byg.Oakleavesorange$BlockCustom
        net.potionstudios.byg.Oakleavesred$BlockCustom
        net.potionstudios.byg.Orangebirchleaves$BlockCustom
        net.potionstudios.byg.Orchardleaves$BlockCustom
        net.potionstudios.byg.Orchardleavesapple$BlockCustom
        net.potionstudios.byg.Orchardleavesflowering$BlockCustom
        net.potionstudios.byg.Palm_leaves$BlockCustom
        net.potionstudios.byg.Paloverdeleaves$BlockCustom
        net.potionstudios.byg.Paloverdeleavesflowering$BlockCustom
        net.potionstudios.byg.Pineleaves$BlockCustom
        net.potionstudios.byg.Rainboweucalyptusleaves$BlockCustom
        net.potionstudios.byg.Redbirchleaves$BlockCustom
        net.potionstudios.byg.Redwoodleaves$BlockCustom
        net.potionstudios.byg.Rowanberryleaves$BlockCustom
        net.potionstudios.byg.Rowanleaves$BlockCustom
        net.potionstudios.byg.Silver_apple_white_skyris_leaves$BlockCustom
        net.potionstudios.byg.Skyrisleaves$BlockCustom
        net.potionstudios.byg.Skyrisleavesgreenapple$BlockCustom
        net.potionstudios.byg.Spruceleavesblue$BlockCustom
        net.potionstudios.byg.Spruceleavesorange$BlockCustom
        net.potionstudios.byg.Spruceleavesred$BlockCustom
        net.potionstudios.byg.Spruceleavesyellow$BlockCustom
        net.potionstudios.byg.Stellataleavespink$BlockCustom
        net.potionstudios.byg.Stellataleaveswhite$BlockCustom
        net.potionstudios.byg.White_skyris_leaves$BlockCustom
        net.potionstudios.byg.Willowleaves$BlockCustom
        net.potionstudios.byg.Witchhazelleaves$BlockCustom
        net.potionstudios.byg.Witchhazelleavesblooming$BlockCustom
        net.potionstudios.byg.Zelkovaleaves$BlockCustom

And the following to grassClassesWhitelist:

        net.potionstudios.byg.Meadowgrass$BlockCustom
        net.potionstudios.byg.Pasturegrass$BlockCustom
        net.potionstudios.byg.Peatgrass$BlockCustom
        net.potionstudios.byg.Rocky_grass$BlockCustom
        net.potionstudios.byg.Sandygrass$BlockCustom

And the following to grassModels:

        byg:block/meadowgrass,up
        byg:block/pasturegrass,up
        byg:block/peatgrass,up
        byg:block/rocky_grass,up
        byg:block/sandygrass,up

The Sonoran Cactus won't work so don't bother with that. I recommend turning off the cactus in the BF config completely for consistency, looks better anyway.