Added common configuration file with the following exposed values: graceEnabled, graceLength and keyRefillAir. This has not been tested in LAN.

src/main/java/com/micle/loginprotection/LoginProtection.java

@@ -1,6 +1,7 @@
 package com.micle.loginprotection;
 
 import com.micle.loginprotection.data.ProtectedPlayers;
+import com.micle.loginprotection.setup.Config;
 import com.micle.loginprotection.setup.Registration;
 import net.minecraft.resources.ResourceLocation;
 import net.minecraftforge.common.MinecraftForge;
@@ -24,6 +25,7 @@ public class LoginProtection {
 
     public LoginProtection() {
         Registration.register();
+        Config.init();
 
         MinecraftForge.EVENT_BUS.register(this);
     }

src/main/java/com/micle/loginprotection/data/ProtectedPlayer.java

@@ -1,5 +1,7 @@
 package com.micle.loginprotection.data;
 
+import com.micle.loginprotection.setup.Config;
+
 import java.util.UUID;
 
 public class ProtectedPlayer {
@@ -9,7 +11,7 @@ public class ProtectedPlayer {
 
     public ProtectedPlayer(UUID player_uuid) {
         this.player_uuid = player_uuid;
-        this.grace_period = 400;
+        this.grace_period = (Config.GRACE_LENGTH.get() * 40);
         this.is_loading = true;
     }
 

src/main/java/com/micle/loginprotection/data/ProtectedPlayers.java

@@ -1,5 +1,6 @@
 package com.micle.loginprotection.data;
 
+import com.micle.loginprotection.setup.Config;
 import net.minecraft.network.chat.TextComponent;
 import net.minecraft.server.level.ServerPlayer;
 import net.minecraftforge.fmllegacy.server.ServerLifecycleHooks;
@@ -38,7 +39,7 @@ public class ProtectedPlayers {
         protected_players.remove(protected_player);
 
         ServerPlayer player = ServerLifecycleHooks.getCurrentServer().getPlayerList().getPlayer(player_uuid);
-        if (player == null) { return; }
+        if (player == null || !Config.GRACE_ENABLED.get()) { return; }
         player.sendMessage(new TextComponent("Grace Period over!"), player_uuid);
     }
 

src/main/java/com/micle/loginprotection/network/C2SKeyPress.java

@@ -1,6 +1,7 @@
 package com.micle.loginprotection.network;
 
 import com.micle.loginprotection.LoginProtection;
+import com.micle.loginprotection.setup.Config;
 import net.minecraft.network.FriendlyByteBuf;
 import net.minecraft.network.chat.TextComponent;
 import net.minecraft.server.level.ServerPlayer;
@@ -25,9 +26,13 @@ public class C2SKeyPress {
             if (LoginProtection.protected_players.getPlayer(sender.getUUID()) == null) { return; }
             if (!LoginProtection.protected_players.getPlayer(sender.getUUID()).isLoading()) { return; }
 
+            if (!Config.GRACE_ENABLED.get()) {
+                LoginProtection.protected_players.removePlayer(sender.getUUID());
+            } else {
                 LoginProtection.protected_players.getPlayer(sender.getUUID()).setLoading(false);
                 sender.sendMessage(new TextComponent("Grace Period started!"), sender.getUUID());
-            if (sender.isInWater()) {
+            }
+            if (sender.isInWater() && Config.KEY_REFILL_AIR.get()) {
                 sender.setAirSupply(sender.getMaxAirSupply());
             }
         });

src/main/java/com/micle/loginprotection/setup/Config.java

@@ -0,0 +1,41 @@
+package com.micle.loginprotection.setup;
+
+import net.minecraftforge.common.ForgeConfigSpec;
+import net.minecraftforge.fml.ModLoadingContext;
+import net.minecraftforge.fml.config.ModConfig;
+
+public class Config {
+    public static ForgeConfigSpec COMMON_CONFIG;
+
+    public static ForgeConfigSpec.BooleanValue GRACE_ENABLED;
+    public static ForgeConfigSpec.IntValue GRACE_LENGTH;
+
+    public static ForgeConfigSpec.BooleanValue KEY_REFILL_AIR;
+
+    public static void init() {
+        initCommon();
+
+        ModLoadingContext.get().registerConfig(ModConfig.Type.COMMON, COMMON_CONFIG);
+    }
+
+    private static void initCommon() {
+        ForgeConfigSpec.Builder builder = new ForgeConfigSpec.Builder();
+
+        builder.comment("Grace period settings").push("grace");
+        GRACE_ENABLED = builder
+                .comment("Whether a player receives a grace period after becoming active or not.")
+                .define("graceEnabled", true);
+        GRACE_LENGTH = builder
+                .comment("How long the grace period lasts for a player in seconds.")
+                .defineInRange("graceLength", 10, 1, Integer.MAX_VALUE/40);
+        builder.pop();
+
+        builder.comment("Key settings").push("key");
+        KEY_REFILL_AIR = builder
+                .comment("Whether a player's air supply gets refilled upon becoming active.")
+                .define("keyRefillAir", true);
+        builder.pop();
+
+        COMMON_CONFIG = builder.build();
+    }
+}