It is still available, you just need to add it back to the View menu. Tools + Customize, Commands tab, Menu bar = View. Select the menu item in Controls where you want to insert it, say the bottom one. Then Add Command, Category = View, Commands = Tab Order.
Your memory of this command does sound a bit hazy, it was never on the Format menu and its never been a dialog. If you want to put it on the Format menu then you can, just pick the Format menu bar in the Commands tab. You can't make it a dialog though, it numbers the controls in the designer view. Some odds that you actually remembered the View + Document Outline command.