Do I REALLY need Windows 10 to run Photoshop?

>>Do I REALLY need Windows 10 to run Photoshop?

No.  As @John T Smith  has wrote there are older Photoshop version that runs on Windows 7.

But I would strongly advise against continuing to use Windows 7 or 8. Microsoft has stopped servicing both versions. There haven't been released any updates or security patches for Windows 7 (and version 8 ) for years. You are taking a very high risk if you connect the PC to the internet.

As long as the computer is used without a connection to the Internet, this may be not a problem.
But since you want to install Photoshop CC, you absolutely need a connection to the Internet. 
Therefore the urgent advice: before you think about Photoshop, switch your operating system to Windows 10.

No - https://helpx.adobe.com/creative-cloud/system-requirements.html
-Win7 is dead https://www.microsoft.com/en-us/windows/windows-7-end-of-life-support-information
-Win8 is also no longer supported https://support.microsoft.com/en-us/help/4580358/windows-8-1-update