An extended kernel is a community-made modification to the core of Windows (NT kernel and system DLLs) that adds new API functions from newer Windows versions, allowing newer software to run on an outdated OS.

VxKex is an API extension set for Windows 7 that allows some Windows 8, 8.1, and 10 applications to run on the older OS. The developer specifically stated that VxKex was for Windows 7 only and would not work on Windows 8.1. However, has recently been introduced in the "Vxkex Next" project. This is a promising development that could provide an alternative to a full kernel replacement.

Thanks to ongoing work by developers like win32 (and others on MSFN), the kernel now allows many “Windows 10-only” applications to run on 8.1:

When Microsoft ended extended support for Windows 8.1 on January 10, 2023, it ceased providing security updates and technical support for the general public. This marks the beginning of the end for software compatibility. Companies like Google and Mozilla inevitably drop support for outdated operating systems, requiring users to upgrade to maintain security.