Google looks to have rolled out material theme icons to Search on the desktop version of the website for anyone who uses it. These icons can be spotted up towards the top of the page above the very first search result in the list and are simply additions to the labels that were already there prior to the icons popping up.
Though completely aesthetic the icons do provide an extra bit of eye-catching design and it makes things a little more visually pleasing.
One thing that might be easy to miss is that the icons are all fitted with Google's signature colors of red, blue, yellow, and green, though only when that particular icon is selected. For example, when you search for something the first icon that will be highlighted will be the "all" tab, which in this case is now accompanied by a colorful magnifying glass icon.
Once you click on the videos tab however, the "all" icon then switches to a more subtle dark grey color while the video icon becomes the colorful focus. The icons also transition to the blue Google uses once you hover over them with the mouse too. While not colored, you might also notice that clicking on the more button brings up a drop down panel with the additional Google services like maps, flights and more all of which have their own material theme icons too.
In addition to being more visually pleasing the icons can also help those to distinguish the different tabs and what they'll show to the user when clicked. This might be useful for someone who may be using Google for the first time and has never seen which tabs are where.
Adding icons to the labels is certainly a simple change and was probably not seen as a necessity by Google, but it does make sense to have them on the desktop version of the search website to fill up some of the white space that was there before, so more than adding a visual element to help distinguish which tab is which it makes for less empty space on the design of the page. And less empty space generally looks better at least to Google.
Google hasn't made any official statements about the icons being there for all users, but then again it might not be something that Google felt the need to inform users of in the first place. It's also entirely possible that some people wouldn't even notice that the icons were never really there.
In either case the rollout for the icons being part of Search on the desktop version of the website should be on a pretty wide scale if not for literally everyone at this point. If they're not showing up for you, chances are it won't be long before they do.
Google's material theme icons can be found in numerous places on Android handsets in all of Google's apps and services, and more places on desktop than just Search, like the Android Messages app which gained Dark Mode and the Material Theme UI around the middle of August of last year.