Developer API

You have to add this plugin as a dependency or soft dependency. Then you can get the death chest service.

public class TestPlugin extends JavaPlugin {

    @Override
    public void onEnable() {
        DeathChestService load = getServer().getServicesManager().load(DeathChestService.class);
        if (load != null) {
            // Code
        }
    }

}

With the service you can create new death chests.

public class TestPlugin extends JavaPlugin {

    @Override
    public void onEnable() {
        DeathChestService load = getServer().getServicesManager().load(DeathChestService.class);
        if (load != null) {
            DeathChest chest = load.createDeathChest(location, items); // This creates a new chest with the items in the world
        }
    }

}

Or you can use the service in other events.

public class TestPlugin extends JavaPlugin implements Listener {

    private DeathChestService deathChestService;

    @Override
    public void onEnable() {
        this.deathChestService = getServer().getServicesManager().load(DeathChestService.class);
        if (this.deathChestService != null) {
            getServer().getPluginManager().registerEvents(this, this); // Registers only if the service is available
        }
    }

    @EventHandler
    public void onQuit(PlayerQuitEvent event) {
        Player player = event.getPlayer();
        ItemStack[] items = null; // Items code here
        this.deathChestService.createDeathChest(player.getLocation(), items); // This creates a new chest with the items in the world
    }

}