Good morning. For Individual subscriptions, as John_Waller says, only the latest version and the previous version (currently versions 14 and 13) are available for download. Adobe's policy is that it will not be developed for versions of Windows prior to Windows 10 (the latest version of Photoshop cannot be installed on Windows 8.1). We recommend upgrading the OS.
* If you are using a Team subscription, you can generate an installer from the Admin console, but since it is an unsupported version, you can not expect future updates and you can't receive customer care support. Please note that there is use at your own risk.