This is a bizarre myth. It’s like saying that trains, bicycles, planes and legs are redundant because someone invented the car. First, proofreading isn't about only grammar and spelling. There's no software on the market that can run through a piece of text such that, by the time it’s finished, that text is publishable. Why? Because software can't spot a widow or an orphan, or a heading at the wrong level, or non-aligned decimal points, missing page numbers, and repeated text in chapters.
Software won’t spot the fact that the thriller you're reading has one character in a room wearing an outfit, and that two pages later she's wearing a different outfit and hasn't left the room; or that a family with two daughters and two sons in Chapter 5 has three daughters and one son by Chapter 48 with no pregnancy in the storyline. These are problems that professional proofreaders frequently encounter.
Furthermore, technology doesn't always get the spelling and grammar right. What software can do is flag up potential issues so that a human can make logical editorial decisions based on skill, knowledge, style preferences and industry-recognised best practice. There are some great tools out there and many professional proofreaders and editors use them, but using them is about complementing the work done by the brain and eyes, not replacing it.