ChatInputHistory using player prefs for Unity, not perfect but does the job

ChatInputHistory.cs

using System.Collections.Generic;
using UnityEngine;

namespace AGES.Networking.AGES.Networking.Runtime.Client.Scripts.UI.Chat
{
    public class ChatInputHistory : MonoBehaviour
    {
        // Keeps a record of previous player chat input and allows the player to scroll through it
        private readonly List<string> chatHistory = new();

        private int historyIndex;

        public void Start()
        {
            // Load from PlayerPrefs
            if (PlayerPrefs.HasKey("ChatHistory"))
            {
                string[] history = PlayerPrefs.GetString("ChatHistory").Split('|');
                foreach (string s in history)
                {
                    if (string.IsNullOrEmpty(s))
                        continue;

                    // Do not add if already in the list
                    if (chatHistory.Contains(s))
                        continue;

                    chatHistory.Add(s);
                }
            }
        }

        public void Add(string input)
        {
            if (string.IsNullOrEmpty(input))
                return;

            if (chatHistory.Contains(input))
                chatHistory.Remove(input);

            chatHistory.Add(input);
            historyIndex = chatHistory.Count;

            // Save to PlayerPrefs
            string history = "";
            foreach (string s in chatHistory)
            {
                history += $"{s}|";
            }

            PlayerPrefs.SetString("ChatHistory", history);
        }

        public string Previous()
        {
            if (chatHistory.Count == 0)
                return "";

            historyIndex--;
            if (historyIndex < 0)
            {
                historyIndex = chatHistory.Count - 1;
            }

            return chatHistory[historyIndex];
        }

        public string Next()
        {
            if (chatHistory.Count == 0)
                return "";

            historyIndex++;
            if (historyIndex >= chatHistory.Count)
            {
                historyIndex = 0;
            }

            return chatHistory[historyIndex];
        }
    }
}