None of the Google developer-oriented resources are accessible from Iran. For example, you have to use a VPN to download Android Studio or access Google educational materials. Golang is just yet another Google project that has blocked Iran.
My intention was to show that in reality it doesn't matter whether a software is open source or proprietary like sourceforge .golang has nit blocked Iran .I highly doubt anyone among go community and creators would think or know about this issue . it is only blocked because of app engine just like many other non-google websites which use app engine.