Source: @ Thomas Ward and @V2Blast
StackExchange did this, not the AU team. The entire structure and format of the site has changed since that original snapshot you've seen. A hint though: new users still didn't read this when posting leading to the closing of things, etc. so it never really had any major impact on things, probably why SE removed it.
As a note, at least parts of this guidance are still shown in certain parts of the UI. For instance, when suggesting an edit (when you don't have the privilege to edit posts directly), or if you open the "Edit" link in a new tab/window, the "How to Edit" guidance is shown on the right side of the page when the cursor is in the title area, "How to Format" is shown when the cursor is in the body text area, and "How to Tag" is shown when the cursor is in the tag area