Oh wow, they put an essential feature back in. Such heroes! Microsoft’s website literally just says they removed the deselect feature in 2013 for “Graphics Enhancement” or some bull**bleep** like that.
They don’t explain why you would ever want to have multiple “selections” within one selection. I've concluded that's because it does nothing.
Because there is no practical reason for ever wanting to select “overlapping cells” with CTRL (you can only take ONE action at a time against a selection, such as copy>paste or changing font color, etc.) the only reason for removing this essential feature is to entice users with older software versions to pay money and upgrade their license. It’s deliberate sabotage of a previously great set of software.
The plot twist to any of those asking how they F*ed this up, is that it was a completely deliberate internal decision. Just take a look at Microsoft.com's official help page regarding this issue, completely and utterly useless, doesn't describe any new feature or benefit, other than the cell fill color changing. https://support.microsoft.com/en-us/help/2769340/cell-shading-continually-darkens-when-you-hold-down-ctrl-and-repeatedl
To restore essential features which were removed meaninglessly, you must pay! It is absolutely infuriating, and they aren’t fooling anyone. Those **bleep**ing assholes at Microsoft should be gutted like hogs and left to rot.