IDManager.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.LowLevel;
using System;

public class IDManager
{
    Queue<int> restoreID;
    int[] ID2Data;
    int[] data2ID;
    public int count { get; private set; }
    int idIndex;

    public void Initialize(int capacity)
    {
        count = 0;
        idIndex = 0;
        ID2Data = new int[capacity];
        data2ID = new int[capacity];
        restoreID = new Queue<int>(capacity);
    }

    public int GetDataIndex(int id) => ID2Data[id];

    public int GetNewID()
    {
        var id = 0;
        if (restoreID.Count != 0)
        {
            id = restoreID.Dequeue();
            ID2Data[id] = count;
            data2ID[count] = id; 
        }
        else
        {
            id = idIndex;
            ID2Data[id] = count;
            data2ID[count] = id;
            idIndex += 1;
        }

        count++;
        return id;
    }

    public void RestoreID(int id)
    {
        count--;
        var removeDataIndex = ID2Data[id];
        var latestIndex = count;
        var newTargetIndex = data2ID[latestIndex];

        data2ID[removeDataIndex] = data2ID[latestIndex];
        ID2Data[newTargetIndex] = removeDataIndex;

        if ( count != id )
        {
            restoreID.Enqueue(id);
        }
    }
}

PlayerLoopExtensions.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.LowLevel;
using System;

public static class PlayerLoopExtensions
{
    public static void Add(this PlayerLoopSystem current, PlayerLoopSystem newLoopSystem, Type order)
    {
        var updateIndex = SubsystemIndex(current, order);
        var updateSubSystem = current.subSystemList[updateIndex];
        var updateSystems = new List<PlayerLoopSystem>(updateSubSystem.subSystemList);
        updateSystems.Add(newLoopSystem);
        updateSubSystem.subSystemList = updateSystems.ToArray();

        current.subSystemList[updateIndex] = updateSubSystem;
    }

    public static void Remove(this PlayerLoopSystem current, PlayerLoopSystem loopSystem, Type order)
    {
        var updateIndex = SubsystemIndex(current, order);
        var updateSubSystem = current.subSystemList[updateIndex];
        var updateSystems = new List<PlayerLoopSystem>(updateSubSystem.subSystemList);
        updateSystems.Remove(loopSystem);
        updateSubSystem.subSystemList = updateSystems.ToArray();

        current.subSystemList[updateIndex] = updateSubSystem;
    }

    static int SubsystemIndex(PlayerLoopSystem current, Type order) => Array.FindIndex(current.subSystemList, c => c.type == order);
}