Plywood only "gives" if you apply an excessive load 2 layers of 5/8" plywood over joists on 16 inch centers is more than adequate for a tiled floor using thinset, particularly if using Ditra. A backsplash takes NO load, so it will NOT flex so doesn't really need a decoupler like Ditra but I WOULD be tempted to use Scluter KERDI on whatever substrate I was tiling.

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