Warn about potentially unstable plugins/server software

api/src/main/java/us/myles/ViaVersion/api/platform/ViaPlatform.java

@@ -31,8 +31,11 @@ import us.myles.ViaVersion.api.command.ViaCommandSender;
 import us.myles.ViaVersion.api.configuration.ConfigurationProvider;
 import us.myles.ViaVersion.api.data.UserConnection;
 import us.myles.ViaVersion.protocols.base.ProtocolInfo;
+import us.myles.ViaVersion.util.UnsupportedSoftware;
 
 import java.io.File;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.UUID;
 import java.util.logging.Logger;
 
@@ -224,4 +227,14 @@ public interface ViaPlatform<T> {
     default ViaConnectionManager getConnectionManager() {
         return Via.getManager().getConnectionManager();
     }
+
+    /**
+     * Returns an immutable collection of classes to be checked as unsupported software with their software name.
+     * If any of the classes exist at runtime, a warning about their potential instability will be given to the console.
+     *
+     * @return immutable collection of unsupported software to be checked
+     */
+    default Collection<UnsupportedSoftware> getUnsupportedSoftwareClasses() {
+        return Collections.emptyList();
+    }
 }

api/src/main/java/us/myles/ViaVersion/util/UnsupportedSoftware.java

@@ -0,0 +1,53 @@
+/*
+ * This file is part of ViaVersion - https://github.com/ViaVersion/ViaVersion
+ * Copyright (C) 2016-2021 ViaVersion and contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package us.myles.ViaVersion.util;
+
+public final class UnsupportedSoftware {
+
+    private final String name;
+    private final String className;
+    private final String reason;
+
+    public UnsupportedSoftware(String name, String className, String reason) {
+        this.name = name;
+        this.className = className;
+        this.reason = reason;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getClassName() {
+        return className;
+    }
+
+    public String getReason() {
+        return reason;
+    }
+
+    public static final class Reason {
+
+        public static final String DANGEROUS_SERVER_SOFTWARE = "You are using server software that - outside of possibly breaking ViaVersion - can also cause severe damage to your server's integrity as a whole.";
+    }
+}

bukkit/src/main/java/us/myles/ViaVersion/ViaVersionPlugin.java

@@ -42,8 +42,11 @@ import us.myles.ViaVersion.bukkit.platform.BukkitViaLoader;
 import us.myles.ViaVersion.bukkit.util.NMSUtil;
 import us.myles.ViaVersion.dump.PluginInfo;
 import us.myles.ViaVersion.util.GsonUtil;
+import us.myles.ViaVersion.util.UnsupportedSoftware;
 
 import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.UUID;
 
@@ -294,6 +297,13 @@ public class ViaVersionPlugin extends JavaPlugin implements ViaPlatform<Player>
         return api;
     }
 
+    @Override
+    public final Collection<UnsupportedSoftware> getUnsupportedSoftwareClasses() {
+        List<UnsupportedSoftware> list = new ArrayList<>(ViaPlatform.super.getUnsupportedSoftwareClasses());
+        list.add(new UnsupportedSoftware("Yatopia", "org.yatopiamc.yatopia.server.YatopiaConfig", UnsupportedSoftware.Reason.DANGEROUS_SERVER_SOFTWARE));
+        return Collections.unmodifiableList(list);
+    }
+
     public boolean isCompatSpigotBuild() {
         return compatSpigotBuild;
     }

common/src/main/java/us/myles/ViaVersion/ViaManagerImpl.java

@@ -35,6 +35,7 @@ import us.myles.ViaVersion.commands.ViaCommandHandler;
 import us.myles.ViaVersion.protocols.protocol1_13to1_12_2.TabCompleteThread;
 import us.myles.ViaVersion.protocols.protocol1_9to1_8.ViaIdleThread;
 import us.myles.ViaVersion.update.UpdateUtil;
+import us.myles.ViaVersion.util.UnsupportedSoftware;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -137,6 +138,9 @@ public class ViaManagerImpl implements ViaManager {
             }
         }
 
+        // Check for unsupported plugins/software
+        unsupportedSoftwareWarning();
+
         // Load Listeners / Tasks
         protocolManager.onServerLoaded();
 
@@ -198,6 +202,34 @@ public class ViaManagerImpl implements ViaManager {
         loader.unload();
     }
 
+    private void unsupportedSoftwareWarning() {
+        boolean found = false;
+        for (UnsupportedSoftware software : platform.getUnsupportedSoftwareClasses()) {
+            try {
+                Class.forName(software.getClassName());
+            } catch (ClassNotFoundException ignored) {
+                continue;
+            }
+
+            // Found something
+            if (!found) {
+                platform.getLogger().severe("************************************************");
+                platform.getLogger().severe("You are using unsupported software and may encounter unforeseeable issues.");
+                platform.getLogger().severe("");
+                found = true;
+            }
+
+            platform.getLogger().severe("We strongly advise against using " + software.getName() + ":");
+            platform.getLogger().severe(software.getReason());
+            platform.getLogger().severe("");
+        }
+
+        if (found) {
+            platform.getLogger().severe("We will not provide support in case you encounter issues possibly related to this software.");
+            platform.getLogger().severe("************************************************");
+        }
+    }
+
     @Override
     public ViaPlatform<?> getPlatform() {
         return platform;