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I don't think it has ever been "required".
Now, as the motion was reduced a mm or so, you're right to assume a regular cover for the 2015- models would just sit there without anything to grab onto.
However if dust inside the keys is not an issue and it's only done for cosmetic purposes, I think a set of keyboard stickers would work just fine, albeit not easily interchangeable for folks who use those "Photoshop/Premiere/OSX/Etc" covers.
 
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