Windows 10 Version 1507, codenamed "Threshold 1," is the very first retail release of the Windows 10 operating system. Released to the public on July 29, 2015, it represents the baseline upon which Microsoft built its "Windows as a Service" (WaaS) model.
Downloading the ISO from Microsoft (via Rufus/Fido) is completely legal as long as you have a valid license. Sharing the ISO on file-sharing sites is copyright infringement. Using unactivated Windows is permitted by Microsoft but limits personalization. windows 10 version 1507 iso
Unless you need 1507 for testing legacy software, offline VMs, or embedded systems, use a supported version: Windows 10 Version 1507: The Original Release Windows
Vulnerability: Systems running this version no longer receive security or quality updates, making them highly vulnerable to modern viruses and malware. Download the ISO