StopPhysicsFixedUpdate.cs

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

public class StopPhysicsFixedUpdate 
{
    static PlayerLoopSystem[] originalPlayerLoop, updatedPlayerLoop;

    [RuntimeInitializeOnLoadMethod()]
    static void Init()
    {
        var playerLoop = PlayerLoop.GetDefaultPlayerLoop();

        originalPlayerLoop = playerLoop.subSystemList[2].subSystemList; // Update

        var subSystem = new List<PlayerLoopSystem>(originalPlayerLoop);
        subSystem.RemoveAll(c => c.type == typeof(FixedUpdate.PhysicsFixedUpdate));
        updatedPlayerLoop = subSystem.ToArray();

        PhysicsOFF();
    }

    public static void PhysicsOFF()
    {
        var playerLoop = PlayerLoop.GetDefaultPlayerLoop();
        var sub = playerLoop.subSystemList[2];
        sub.subSystemList = updatedPlayerLoop;
        playerLoop.subSystemList[2] = sub;
        PlayerLoop.SetPlayerLoop(playerLoop);
    }

    public static void PhysicsON()
    {
        var playerLoop = PlayerLoop.GetDefaultPlayerLoop();
        var sub = playerLoop.subSystemList[2];
        sub.subSystemList = originalPlayerLoop;
        playerLoop.subSystemList[2] = sub;
        PlayerLoop.SetPlayerLoop(playerLoop);
    }
}