All the layers, except applications where devs still look for the wrong slash manually.

OSes and modern programming languages have APIs for file navigation for a reason.

On Windows this implies old applications from another era, good old command window, or people that don't know better.

The kernel and ntfs does not care about slash direction. Specific programs like dir might though and honestly if you're on windows just use powershell and avoid legacy cmd stuff.

Use powershell.

Get-ChildItem /w