Because this is an older build, Microsoft does not list it on the primary download page. There are two primary methods to retrieve the official files.
No, Microsoft does not provide an official public-facing archive. The only legitimate archives are for subscribers (MSDN, VLSC) or via the retired Windows ISO Downloader tool from Microsoft (now defunct). Windows 10 Version 1511 Build 10586 Iso Download
If you still need the ISO, follow this process: Because this is an older build, Microsoft does