Honestly, I don't think it has ever been necessary to put a back button on a website. Even in 2001, when I first started making web sites, I never placed a "back" button on the page - because it's redundant within the tool being used to view the site.
Now, there are exceptions - but very, very, few. When I made Flash sites, before using hashtag URLs to maintain the functionality of the back button - if there was no other way for a user to back-track to the view they were just on, I would add a back button. What that means is, if the user could, based on the storyboards, go from say page A to page B, then, using the primary navigation get back to page A...no back button required/desired.
Of course with our ability to do things in a way which no longer breaks the ability of the back button to go back in the browser history - it just ups the logic to not have a back button to navigate from one page to another.
In the case of a "previous" or "cancel" button like those found in surveys or forms, it makes sense. Especially if the button does not actually simply move the browser history back in the stack. For example, we may want clear some data from the form, or add a "jump to where I left off" functionality. Which is not literally the same as "the browser's back function".