Source Code for FallingSnow (https://www.spigotmc.org/resources/92367/)

FallingSnow.java

package com.loohp.fallingsnow;

import net.md_5.bungee.api.ChatColor;
import org.bukkit.Bukkit;
import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.NamespacedKey;
import org.bukkit.block.Block;
import org.bukkit.block.BlockFace;
import org.bukkit.block.data.BlockData;
import org.bukkit.block.data.type.Snow;
import org.bukkit.entity.EntityType;
import org.bukkit.entity.FallingBlock;
import org.bukkit.event.Cancellable;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.BlockPhysicsEvent;
import org.bukkit.event.block.BlockPistonExtendEvent;
import org.bukkit.event.block.BlockPistonRetractEvent;
import org.bukkit.event.entity.EntityChangeBlockEvent;
import org.bukkit.event.entity.EntityDropItemEvent;
import org.bukkit.persistence.PersistentDataContainer;
import org.bukkit.persistence.PersistentDataType;
import org.bukkit.plugin.Plugin;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Consumer;

public class FallingSnow extends JavaPlugin implements Listener {
	
	public static FallingSnow plugin;

	public NamespacedKey fallingSnowTag;
	public PersistentDataType<Byte, Byte> fallingSnowTagType = PersistentDataType.BYTE;
	public byte fallingSnowTagValueFalse = 0;
	public byte fallingSnowTagValueTrue = 1;

	public Set<Block> checkedBlocks = ConcurrentHashMap.newKeySet();

	private static final boolean FOLIA = classExist("io.papermc.paper.threadedregions.scheduler.AsyncScheduler");

	private static boolean classExist(String className) {
		try {
			Class.forName(className);
			return true;
		} catch (ClassNotFoundException e) {
			return false;
		}
	}

	public static void runTaskTimer(Plugin plugin, Runnable runnable, int delay, int period) {
        if (FOLIA) {
		//noinspection rawtypes,unchecked
		Bukkit.getGlobalRegionScheduler().runAtFixedRate(plugin, (Consumer) st -> runnable.run(), delay, period);
        } else {
            	Bukkit.getScheduler().runTaskTimer(plugin, runnable, delay, period);
        }
    }

	public static void runTaskLater(Plugin plugin, Location location, Runnable runnable, int delay) {
		if (FOLIA) {
			//noinspection rawtypes,unchecked
			Bukkit.getRegionScheduler().runDelayed(plugin, location, (Consumer) st -> runnable.run(), delay);
		} else {
			Bukkit.getScheduler().runTaskLater(plugin, runnable, delay);
		}
	}
	
	@Override
	public void onEnable() {
		plugin = this;

		fallingSnowTag = new NamespacedKey(this, "IsFallingSnow");

		getServer().getPluginManager().registerEvents(this, this);
		getServer().getConsoleSender().sendMessage(ChatColor.GREEN + "[FallingSnow] FallingSnow has been enabled.");
		
		runTaskTimer(this, () -> checkedBlocks.clear(), 1, 1);
	}
	
	@EventHandler
	public void onPhysics(BlockPhysicsEvent event) {
		Block block = event.getBlock();
		checkSnowLayer(block);
		checkSnowLayer(block.getRelative(BlockFace.UP));
	}
	
	@EventHandler
	public void onPistonExtend(BlockPistonExtendEvent event) {
		for (Block block : event.getBlocks()) {
			checkSnowLayer(block);
			checkSnowLayer(block.getRelative(BlockFace.UP));
		}
	}
	
	@EventHandler
	public void onPistonRetract(BlockPistonRetractEvent event) {
		for (Block block : event.getBlocks()) {
			checkSnowLayer(block);
			checkSnowLayer(block.getRelative(BlockFace.UP));
		}
	}
	
	public void checkSnowLayer(Block block) {
		if (!checkedBlocks.contains(block) && block.getType().equals(Material.SNOW)) {
			BlockData data = block.getBlockData().clone();
			runTaskLater(this, block.getLocation(), () -> {
				if (block.getType().equals(Material.AIR) && block.getRelative(BlockFace.DOWN).getType().equals(Material.AIR)) {
					FallingBlock fallingBlock = block.getWorld().spawnFallingBlock(block.getLocation().add(0.5, 0, 0.5), data);
					fallingBlock.getPersistentDataContainer().set(fallingSnowTag, fallingSnowTagType, fallingSnowTagValueTrue);
				}
			}, 2);
		}
		checkedBlocks.add(block);
	}
	
	public void handleSnowLayer(FallingBlock fallingBlock, Block original, Cancellable event) {
		BlockData blockData = fallingBlock.getBlockData();
		if (blockData.getMaterial().equals(Material.SNOW)) {
			Snow snow = (Snow) blockData;
			if (original.getType().equals(Material.SNOW)) {
				event.setCancelled(true);
				Snow originalSnow = (Snow) original.getBlockData();
				int rawLayers = originalSnow.getLayers() + snow.getLayers();
				int layers = Math.min(rawLayers, 8);
				originalSnow.setLayers(layers);
				original.setBlockData(originalSnow);
				if (rawLayers > layers) {
					Block up = original.getRelative(BlockFace.UP);
					if (up.getType().equals(Material.AIR)) {
						up.setType(Material.SNOW);
						Snow upSnow = (Snow) up.getBlockData();
						upSnow.setLayers(rawLayers - layers);
						up.setBlockData(upSnow);
					}
				}
			}
		}
	}
	
	@EventHandler
	public void onFallingBlockLanding(EntityChangeBlockEvent event) {
		if (event.getEntityType().equals(EntityType.FALLING_BLOCK)) {
			handleSnowLayer((FallingBlock) event.getEntity(), event.getBlock(), event);
		}
	}
	
	@EventHandler
	public void onFallingBlockPop(EntityDropItemEvent event) {
		if (event.getEntityType().equals(EntityType.FALLING_BLOCK)) {
			FallingBlock fallingBlock = (FallingBlock) event.getEntity();
			PersistentDataContainer dataContainer = fallingBlock.getPersistentDataContainer();
			if (dataContainer.getOrDefault(fallingSnowTag, fallingSnowTagType, fallingSnowTagValueFalse) >= fallingSnowTagValueTrue) {
				event.setCancelled(true);
			}
			handleSnowLayer(fallingBlock, fallingBlock.getLocation().getBlock(), event);
		}
	}

}

Will be fixed on Folia soon