where is option for downloading older version on new creative cloud

The min requirements have changed to no more than 2 versions back.  Adobe expects customers to have at least macOS Mojave or Win10 (v 1809) or higher now.

Please ensure your computer meets or exceeds the minimum system requirements to run Creative Cloud + OS + all other apps you use. Check each one as requirements vary by product.
- https://helpx.adobe.com/creative-cloud/system-requirements.html

Also pay attention to Photoshop's new GPU requirements. See links below.
- https://helpx.adobe.com/photoshop/system-requirements.html
- https://helpx.adobe.com/photoshop/kb/photoshop-cc-gpu-card-faq.html

Check the box to Show Older Apps from your CC Desktop App's Preferences. 

Avatar (top right image) > Preferences > Apps > Settings.

Hope that helps.

Adobe now offers the current and ONE older version... so if the one older version won't run on your computer I don't think you will see the option

Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-the prodesign links go directly to Adobe's file server, so this process
--will only work until Adobe completely removes the old version(s) from the server
.
When you do have a download, be sure to copy it to AT LEAST one external device
-a usb flash drive or hard drive, or write to a DVD