The SDK is free to download and use. You do not need a Visual Studio license to use the command-line tools, headers, and libraries.
If you do not have a strict requirement for Windows 8.1, consider these newer SDKs: windows sdk 8.1 download
Modern versions of Visual Studio (like VS 2019 or VS 2022) allow developers to "target" older platforms. You can write code in a modern IDE but tell the compiler to treat the code as if it were being built for Windows 8.1. To do this, the IDE requires that the Windows SDK 8.1 be installed on the machine. Without it, you cannot select "8.1" as the target platform version in your project properties. The SDK is free to download and use