If you have an active subscription, you can sometimes find older versions (typically only back to 2022 or 2023) by clicking the three dots next to Photoshop in the Creative Cloud Desktop app and selecting "Other versions" .
You can download the full version of the latest Photoshop for a one-week trial. This trial automatically converts to a paid subscription unless cancelled before the 7-day period ends. Adobe Photoshop CC 2015 Download Free Trial Version
If you are looking for this specific version due to its low system requirements or unique features, it included: Why can't I download an older versions of some programs If you have an active subscription, you can