June 26, 2013, during the Microsoft Build conference. Release to Manufacturing (RTM): August 27, 2013.
Although Windows 8.1 is an older operating system, official ISO files can still be sourced for recovery or legacy system maintenance. windows 8.1 download release date
You can still find the Windows 8.1 ISO Download on Microsoft's site. You will need to select your edition (Windows 8.1, K, KN, or N) and language before downloading the 32-bit or 64-bit version. June 26, 2013, during the Microsoft Build conference