Ctrl + “T” (Default Assigned to “Select None”) is not ostensibly an OS-reserved keyboard shortcut like Ctrl +“C”, Ctrl + “V” or Ctrl +“N”, Ctrl + "S"
It is re-assignable to a custom shortcut.
This custom shortcut functions as reassigned during a session of Sketchup.
However on restart of SketchUp, the shortcut reverts to default of “Select None”
This behavior is observed on SketchUp 2018, Windows 10.
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.
+1 for using Shift to inverse or reverse a command. Ctrl+Shift+A is a very logical way to deselect all. To rant a bit, Steam using Shift+Tab top open chat is one of my pet peeves as it clearly means cycle backwards.
Personally I’ve always activated PushPull to empty the selection, and even when I read about “Select None” in some shortcut list I couldn’t make sense of it. I interpreted it as adding nothing to the selection, which is the same as doing nothing at all. Now that PushPull supports pre-select it might be useful to use another shortcut for emptying the selection, but I’m so used to moving my fingers this way.
In any case Ctrl+T should eb available for the user to assign to whatever we want.
It appears that Ctrl + “T” is some kind of ‘reserved’ SU shortcut then.
I had assigned Ctrl + “D” (like in Photoshop) for “Select None”
Another advantage is that it is closer to the left-hand side of the keyboard!
If this is not a bug, then I am interested to know what other key combinations exhibit the same behaviour of allowing reassignment during current session of SU but revert to default behaviour on re-start of application.
So far I have only added additional shortcuts for “Select None”, but not assigned its default to another action. That Ctrl+T does not stick to other commands could be a bug, I have to test it.
Same scenario here.
I had assigned "Select None to Ctrl + “D” since I started using SketchUp.
Was recently attempting to assign Ctrl + “T” to TrueBend when I discovered this weird un-sticky behavior!
The Mac version is actually better than the Windows version in one aspect?? 
(Other than visual appearance)
