Short answer : they are still there, in the "Shared Elements" section of the "Applications" view (where you find your forms, views, agents etc). Don't touch unless you are dealing with a grossly antiquated legacy app.
Long answer : they are sort of deprecated. Now this is an element that I haven't seen in years, maybe since the advent of version 6.5 in, wait, was it 2005 ? I don't even know if they are suitable for the web, but I'd bet no.
As stated in another comment, the easiest way is to start with a Frameset devided in two columns. The left column contains a Page, which itself contains an Outline.
I know, I know, framesets themselves are deprecated, or at least frowned upon by the up-and coming young devs. If you wanted to be really up-to-date, you'd start with an XPage and your "navigator" would end up being a <div>