A popular development pipeline, and one of the benefits of the Windows Subsystem for linux is to use a Linux based developer toolchain, i.e. gcc etc within WSS and then to access those build artifacts from Windows. In earlier versions of Windows 10 (1703 and earlier), you could access the Windows Subsystem for Linux file system via:
C:\Users\%USERNAME%\AppData\Local\lxss
This allowed you to access 'Linux' files from Windows.
Strictly speaking you were not supposed to modify any files there - but it was useful for read access.
Reference: https://github.com/Microsoft/BashOnWindows/issues/402#issuecomment-220585695
Under the Fall Creators Update of Windows 10 (1709), this seems to have disappeared.
Where has it gone? Clearly a workaround is to, as the last stage of the built pipeline copy out of Linux into Windows using the /mnt mount, but not being access the files from windows seems like it must be a problem for many developers.
It seems that in order to support the capability to install several different installations of the Windows Subsystem for Linux, storing the file system in fixed location no longer made sense.
I created a unique file name in the /home/username directory, and then searched for it from Windows.
In my particular case the file system is stored in:
C:\Users\donal\AppData\Local\Packages\CanonicalGroupLimited.UbuntuonWindows_79rhkp1fndgsc\LocalState\rootfs\home\username
It seems likely that the location varies on every installation.