I confirm that it doesn't autoremove. I had to empty /boot on some servers lately.

Anyway sometimes one wants to keep old kernels. I have an old laptop that runs OK with a 3.something kernel and has wierd video sync problems with any newer ones. Ubuntu 16.04 keeps running with that old kernel so I keep booting from that, maybe once or twice per year.

However the proper solution would be pinning a package and autoremoving the others.