feat: PlayerChannelUnregisterEvent (#1686)

* feat: PlayerChannelUnregisterEvent

* style: fix checkstyle issues

api/src/main/java/com/velocitypowered/api/event/player/PlayerChannelUnregisterEvent.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2025 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.event.player;
+
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.proxy.Player;
+import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
+import java.util.List;
+
+/**
+ * This event is fired when a client ({@link Player}) sends a plugin message through the
+ * unregister channel. Velocity will not wait on this event to finish firing.
+ */
+public final class PlayerChannelUnregisterEvent {
+
+  private final Player player;
+  private final List<ChannelIdentifier> channels;
+
+  public PlayerChannelUnregisterEvent(Player player, List<ChannelIdentifier> channels) {
+    this.player = Preconditions.checkNotNull(player, "player");
+    this.channels = Preconditions.checkNotNull(channels, "channels");
+  }
+
+  public Player getPlayer() {
+    return player;
+  }
+
+  public List<ChannelIdentifier> getChannels() {
+    return channels;
+  }
+
+  @Override
+  public String toString() {
+    return "PlayerChannelUnregisterEvent{"
+            + "player=" + player
+            + ", channels=" + channels
+            + '}';
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -24,6 +24,7 @@ import com.mojang.brigadier.suggestion.Suggestion;
 import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.event.player.CookieReceiveEvent;
 import com.velocitypowered.api.event.player.PlayerChannelRegisterEvent;
+import com.velocitypowered.api.event.player.PlayerChannelUnregisterEvent;
 import com.velocitypowered.api.event.player.PlayerClientBrandEvent;
 import com.velocitypowered.api.event.player.TabCompleteEvent;
 import com.velocitypowered.api.event.player.configuration.PlayerEnteredConfigurationEvent;
@@ -318,8 +319,12 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
                 new PlayerChannelRegisterEvent(player, ImmutableList.copyOf(channels)));
         backendConn.write(packet.retain());
       } else if (PluginMessageUtil.isUnregister(packet)) {
-        player.getClientsideChannels()
+        List<ChannelIdentifier> channels =
-            .removeAll(PluginMessageUtil.getChannels(0, packet, this.player.getProtocolVersion()));
+            PluginMessageUtil.getChannels(0, packet, this.player.getProtocolVersion());
+        player.getClientsideChannels().removeAll(channels);
+        server.getEventManager()
+            .fireAndForget(
+                new PlayerChannelUnregisterEvent(player, ImmutableList.copyOf(channels)));
         backendConn.write(packet.retain());
       } else if (PluginMessageUtil.isMcBrand(packet)) {
         String brand = PluginMessageUtil.readBrandMessage(packet.content());