Xe could hyperlink you to Dustin Kirkland's call for feedback here on Hacker News only a short while ago, where this subject came up several times. This is a well-known Ubuntu problem, and M. Kirkland xyrself said that taking purge-old-kernels out of the byobu package and putting it somewhere more obvious was a good idea.

* https://news.ycombinator.com/item?id=14002821

Of course, this is an Ubuntu upgrade problem relating to Linux. It is specifically about kernel image files. So it has very little application to the Windows NT Linux Subsystem, which as we know uses the Windows NT kernel.