I remember when Virtualbox was removed from debian, and I remember the reasons. That's why I was surprised when I saw a recent message to the list where a user tried to install it. Searching for "virtualbox" indeed shows me that it is available in stretch-backports and sid: https://packages.debian.org/virtualbox&exact=1
Has something changed that makes virtualbox workable again, or has it *always* been available in sid?