As of Photoshop CC 2019 (20.0.8), Adobe no longer offers the ability to download older versions of Photoshop. Even if you have a previous version of Photoshop installed on your computer, you will be unable to open it once you update to the latest version.
If you’re a Creative Cloud subscriber, you always have access to the latest version of Photoshop. If you’re not a subscriber, you can purchase Photoshop CC 2019 from Adobe’s website.
If you need to use an older version of Photoshop for compatibility reasons, Adobe recommends using the Creative Cloud desktop app to download and install the older version.
So, if you’re looking to download an older version of Photoshop, unfortunately, you’re out of luck. The only way to use an older version is if you have a Creative Cloud subscription or if you purchase the latest version.