ICustomYieldInstruction.cs

using System.Collections;
using UnityEngine;

namespace UnityEngine
{
    public interface ICustomYieldInstruction : IEnumerator
    {
        public bool keepWaiting { get; }

        object IEnumerator.Current => null;

        bool IEnumerator.MoveNext() => keepWaiting;

        void IEnumerator.Reset()
        {
        }
    }
}


public class WaitFiveFrames : ICustomYieldInstruction
{
    private int i;
    public bool keepWaiting => i++ < 5;
}

public class Test : MonoBehaviour
{
    public void Awake()
    {
        StartCoroutine(Coro());
    }

    private IEnumerator Coro()
    {
        Debug.Log(Time.frameCount);
        yield return new WaitFiveFrames();
        Debug.Log(Time.frameCount);
    }
}