Add warning for deprecated commands

CommandsText.cs

@@ -67,6 +67,7 @@ internal class CommandsText {
         if (msgsplit[0].StartsWith(CommandPrefix, StringComparison.OrdinalIgnoreCase)) {
             var cmdBase = msgsplit[0][3..];
             if (_commands.ContainsKey(cmdBase)) {
+                NoiTheCat.TextCommandRemovalWarning.Intercept(message, channel.Guild.Id);
                 Program.Log("Command invoked", $"{channel.Guild.Name}/{message.Author} {message.Content}");
                 try {
                     await _commands[cmdBase](channel, (SocketGuildUser)message.Author, message).ConfigureAwait(false);

TextCommandRemovalWarning.cs

@@ -0,0 +1,41 @@
+namespace NoiTheCat;
+static class TextCommandRemovalWarning {
+    public const string StopUsingTextCommands = ":warning: **Reminder**: Text-based commands will be phased out by the end of August. " +
+        "Please switch to using slash commands. For details on their usage, use this bot's `/help` command.";
+    private static readonly RateLimit<ulong> _warnedList = new(8 * 60 * 60); // 8 hours
+
+    public static void Intercept(SocketMessage msg, ulong gid) {
+        lock (_warnedList) {
+            if (!_warnedList.IsPermitted(gid)) return;
+            try {
+                msg.Channel.SendMessageAsync(StopUsingTextCommands).GetAwaiter().GetResult();
+            } catch (Exception e) {
+                _warnedList.Reset(gid);
+                Console.WriteLine(e.ToString());
+            }
+        }
+    }
+    private class RateLimit<T> where T : notnull {
+        private const int DefaultTimeout = 20;
+        public int Timeout { get; }
+        private Dictionary<T, DateTime> Entries { get; } = new Dictionary<T, DateTime>();
+        public RateLimit() : this(DefaultTimeout) { }
+        public RateLimit(int timeout) {
+            if (timeout < 0) throw new ArgumentOutOfRangeException(nameof(timeout), "Timeout valie cannot be negative.");
+            Timeout = timeout;
+        }
+        public bool IsPermitted(T value) {
+            if (Timeout == 0) return true;
+            var now = DateTime.Now;
+            var expired = Entries.Where(x => x.Value.AddSeconds(Timeout) <= now).Select(x => x.Key).ToList();
+            foreach (var item in expired) Entries.Remove(item);
+            if (Entries.ContainsKey(value)) return false;
+            else {
+                Entries.Add(value, DateTime.Now);
+                return true;
+            }
+        }
+        public bool Reset(T value) => Entries.Remove(value);
+    }
+
+}