NeoForge made those two fields public when reintroduce the patch.

Author: 3TUSK

src/main/java/org/teacon/xkdeco/mixin/forge/FenceGateBlockAccess.java

@@ -8,7 +8,6 @@
 import org.teacon.xkdeco.block.XKDBlock;
 import org.teacon.xkdeco.duck.XKDPlayer;
 import org.teacon.xkdeco.init.MimicWallsLoader;
-import org.teacon.xkdeco.mixin.forge.FenceGateBlockAccess;
 
 import net.minecraft.core.BlockPos;
 import net.minecraft.core.Direction;
@@ -51,7 +50,7 @@ public static boolean isLadder(BlockState blockState, LevelReader world, BlockPo
 	}
 
 	public static SoundEvent getFenceGateSound(FenceGateBlock block, boolean open) {
-		return open ? ((FenceGateBlockAccess) block).getOpenSound() : ((FenceGateBlockAccess) block).getCloseSound();
+		return open ? block.openSound : block.closeSound;
 	}
 
 	public static void moveEntity(XKDPlayer player, Entity entity, Vec3 pPos) {

src/main/java/org/teacon/xkdeco/util/CommonProxy.java

@@ -8,8 +8,7 @@
 		"ReloadableServerResourcesMixin",
 		"air_duct.EntityMixin",
 		"air_duct.LivingEntityMixin",
-		"air_duct.PlayerMixin",
-		"forge.FenceGateBlockAccess"
+		"air_duct.PlayerMixin"
 	],
 	"client": [
 		"air_duct.PlayerRendererMixin",