Why is my version of CCleaner no longer compatible with Windows 10?

This is because Microsoft updated how it uses folders in Windows 10:

Cleaner v5.45 and below clean a folder that Microsoft previously used to store old Windows installation files (called the .old folder). Microsoft Windows now uses this folder to store files it needs, so cleaning it could cause potential problems. 

What happens if you run CCleaner v.5.45 or below on Windows 10: 

Microsoft displays a notification asking you to update if you try to run an incompatible version of CCleaner on Windows 10. If you’re using an older version of CCleaner and see this notification, please update it to the latest version

We always recommend that CCleaner users keep their software up to date. Here’s why.