SteamAPI_Shutdown actually does something now.

dll/dll.cpp

@@ -135,6 +135,7 @@ Steam_Client *get_steam_clientserver_old()
 S_API void * S_CALLTYPE SteamInternal_CreateInterface( const char *ver )
 {
     PRINT_DEBUG("SteamInternal_CreateInterface %s\n", ver);
+    if (!get_steam_client()->user_logged_in) return NULL;
 
     if (strstr(ver, "SteamClient") == ver) {
         void *steam_client;
@@ -217,6 +218,7 @@ S_API bool S_CALLTYPE SteamAPI_InitAnonymousUser()
 S_API void S_CALLTYPE SteamAPI_Shutdown()
 {
     PRINT_DEBUG("SteamAPI_Shutdown\n");
+    get_steam_client()->clientShutdown();
 }
 
 // SteamAPI_RestartAppIfNecessary ensures that your executable was launched through Steam.

dll/steam_client.cpp

@@ -421,6 +421,11 @@ void Steam_Client::serverShutdown()
     server_init = false;
 }
 
+void Steam_Client::clientShutdown()
+{
+    user_logged_in = false;
+}
+
 void Steam_Client::setAppID(uint32 appid)
 {
     std::lock_guard<std::recursive_mutex> lock(global_mutex);

dll/steam_client.h

@@ -274,6 +274,7 @@ public:
     void userLogIn();
     void serverInit();
     void serverShutdown();
+    void clientShutdown();
     bool IsServerInit();
     bool IsUserLogIn();
 };