Yes, the clip property has been deprecated by the clip-path property. You are correct that it should not be commented out but left in uncommented to support older browsers. From what I understand clip-path is not well supported and thus also required another step to add in some SVG to each page that uses this technique. The technique is discussed here: http://unobfuscated.blogspot.com/2015/01/updated-offscreen-techniques.html#!/2015/01/updated-offscreen-techniques.html
Regarding WebAIM’s recommendation to just position text off-screen and not use clip or clip-path – we generally don’t recommend that as it causes the screen to jump on mobile device in particular on iOS as the screen reader VoiceOver tries to draw a rectangle off the left edge of the screen.
As you might imagine there is no perfect solution and generally it our recommendation is to not use off-screen text. There may be other ways to provide access to the same information such as putting the text on-screen or on-screen collapsed so it may benefit all users including users with low vision. Other options exist as well to use aria-describedby or aria-labelledby to associate content elsewhere on-screen with a control. As the WebAIM article points out – interactive content should not be off-screen but appear on-screen when focused.
In regards to updating our best practice with this information, it is on the list to be addressed later this week or next. A recent security upgrade caused an issue with editing certain types of examples in AMP and so the update of the best practice was delayed until the issue is addressed.
Chief Accessibility Officer