According to Microsoft, “Going forward, we’ll be decoupling Search and Cortana in the taskbar. This will enable each experience to innovate independently to best serve their target audiences and use cases.” The company claims that this will provide users with the best search experience depending on their needs, whether it be search the good old fashioned way, or by using Cortana.
Note that this is currently being tested in the Windows 10 Insider Preview Build 18317. This means that this is pretty much a beta feature that has yet to be applied to the main release of Windows 10. It also means that based on additional feedback, there is a chance that they might scrap this idea. Whether or not this is a good idea remains to be seen, but if you were never a fan of search and Cortana integrating with each other, perhaps this is an update that you can keep an eye out for in the future.