Not only is software-defined storage a thing, it is becoming common enough to start considering your options when looking to adopt this particular type of storage infrastructure. And there are plenty of options to pick from.
When you first look at the software-defined storage market, the decisions you make will help you get the most value from the option you end up choosing. Since the storage software at the heart of SDS has become sophisticated enough that many companies can work with just about any piece of commodity hardware the IT department chooses, that hardware is one of many ways in which your organization can get maximum value. Of course, you could choose to run your SDS entirely virtual; in which case, the choice of hardware is moot -- your SDS implementation would be on virtual machines and be one degree removed from the hardware, be it commodity or specialized.
Of course, as you decide how to implement SDS in your organization, you should also look at the software-defined storage market itself. Why has software-defined storage become a thing in the first place? Make sure to take a closer look at the pros and cons of the major ways you can implement SDS -- based on a vendor's recommended or specialized hardware, installed on an organization's own commodity hardware or installed as a completely virtual storage play.
Finally, you'll want to consider software-defined storage in the overall context of software-defined architecture and hyper-converged infrastructure (HCI). This area is fraught with unclear terminology so make sure you understand what makes something "software defined" before trying to understand how SDS gets used within an HCI system.