Possibly the biggest hint is /etc/apk/world, the "worldfile", or the list of explicitly installed packages. It's even the same verbiage Gentoo uses (albeit a different location - Gentoo stores it somewhere under /var)
Either way, I've been watching that s6 project for a couple years now and am looking forward to when that happens. Is there any progress on that front? I hadn't heard that the replacement might not actually be s6.