It's possible to directly download the ISO file for Windows 10 1809 from Microsoft's servers if you have a valid product key.
For a more streamlined experience, the open-source tool Rufus allows you to download older ISOs directly from Microsoft’s servers through its built-in download script. This is the preferred method for creating bootable USB media for version 1809. Important Safety and Lifecycle Notes download exclusive windows 10 1809
Shortly after its initial release in October 2018, Microsoft discovered a critical bug where the update would, in rare cases, delete user files in the Documents folder during the upgrade process. Consequently, Microsoft halted the rollout and removed the installation media from their public servers. This immediate removal created a scenario where the ISO files became difficult to find for the average user, turning a standard update into a sought-after "exclusive" file found only in specific archives or enterprise portals. It's possible to directly download the ISO file