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So long ago that you needn't bother. Centuries ago. Why? Well, another dialect took over perhaps? BTW, in some parts of Russia they still pronounce -ogo as -ogo. But never -ago, it's absolete. You...

As in святаго духа? It's pronounced -ago, but nobody uses it. In fact, I wasn't aware of this form until I turned 30 something I suppose. It's not used any more.