It has always been used for that. Ctrl+C and Ctrl+V (etc.) are not OS-reserved, they are assigned by applications or their application GUI toolkits if there is something copyable, and they are used for this purpose by convention. I am not aware of a convention for Ctrl+T. SketchUp has always used it for the inverse of Ctrl+A (although I would prefer invers actions were consistently like Ctrl+Shift+A (like in many OSS applications).
I agree, an application should not conflict with OS shortcuts. But there is no standard to keep them apart: For example on modern Linux OS, the Super (Windows) modifier is only used for OS shortcuts and Alt is only used for window management (moving windows). However that is different in the Windows world. The rules are often proven by exceptions like SketchUp which makes heavy use of the Alt modifier key.
Generally Ctrl and Shift are save to be used by applications. But since shortcuts have less discoverability than menus, they should not surprise users but follow conventions.