Deselect CPU in affinity setting for OS Only use?
Changing CPU affinity was never a common practice but did have it's uses on WindowsNT and later on Windows Server 2000/3,

The main issue was that processor load could be misaligned on multiprocessor systems and this allowed for freeing up resources. This could also be helpful on systems that where not dedicated to running SQL Server. So yes this could be beneficial in some edge cases. Old Small Business Servers come to mind.

On later versions of Windows the OS is more likely to distribute load between processor so for performance reasons this will not help but can be beneficial on servers running multiple instances if you want to limit those to specific CPU or NUMA node