Unity Pendulum script

Pendulum.cs

using UnityEngine;

public class Pendulum : MonoBehaviour
{
    [Header("Settings")]
    [SerializeField] private float _Speed = 1;
    [SerializeField] private float _Distance = 20;

    [Header("Offset")]
    [SerializeField] private Vector3 _RotationOffset = Vector3.zero;
    [SerializeField] private bool _SetCurrentRotationAsOffset = true;

    enum AxisOptions { X, Y, Z }
    [SerializeField] private AxisOptions _Axis = AxisOptions.X;

    private void Start()
    {
        if (_SetCurrentRotationAsOffset)
            _RotationOffset = transform.eulerAngles;
    }

    void Update()
    {
        float angle = _Distance * Mathf.Sin(Time.time * _Speed);
        switch (_Axis)
        {
            case AxisOptions.X:
                transform.localRotation = Quaternion.Euler(_RotationOffset.x + angle, _RotationOffset.y, _RotationOffset.z);
                break;
            case AxisOptions.Y:
                transform.localRotation = Quaternion.Euler(_RotationOffset.x, _RotationOffset.y + angle, _RotationOffset.z);
                break;
            case AxisOptions.Z:
                transform.localRotation = Quaternion.Euler(_RotationOffset.x, _RotationOffset.y, _RotationOffset.z + angle);
                break;
        }
    }
}

Thanks,
A version with more control over the angle + an optional period on the offset, so the swing can feel more natural for some organic cases.

    using UnityEngine;

    /// <summary>
    /// From https://gist.github.com/MarcelvanDuijnDev/c207c2e326ceda50791e4c680c898f2a
    /// </summary>
    [ExecuteAlways]
    public class Pendulum : MonoBehaviour
    {
        [Header("Settings")]
        [SerializeField] private float _Speed = 1;
        [SerializeField] private float _Distance = 20;

        [Header("Offset")]
        [SerializeField] private Vector3 _RotationOffset = Vector3.zero;
        [SerializeField] private bool _SetCurrentRotationAsOffset = true;

        [SerializeField] private Vector3 _Axis = Vector3.right;

        public bool HasPeriodicOffset;

        public bool DebugInEditMode;

        private void Start()
        {
            if (_SetCurrentRotationAsOffset)
                _RotationOffset = transform.eulerAngles;
        }

        void Update()
        {
            if(!Application.isPlaying && !DebugInEditMode) return;
            
            float angle = _Distance * Mathf.Sin(Time.time * _Speed);
            float periodOffset = HasPeriodicOffset ? Mathf.Cos(Time.time * _Speed) : 1;
            transform.localRotation = Quaternion.Euler(_RotationOffset.x * periodOffset + angle * _Axis.x, _RotationOffset.y * periodOffset + angle * _Axis.y, _RotationOffset.z * periodOffset + angle * _Axis.z);
        }
    }

Something like Offset (0,0,5) and Axis (1, 0, 0.2) is nice in our case for instance.