This is nonsense. It certainly used to be true, but it isn't anymore.
Slackware has fallen prey to trying to compete with Ubuntu and ilk. Now you need to have a full install and the community isn't willing to support uou if you have anything less than that.
This used to be fine, but now they have made it so that there are crazy dependencies. You can't install mplayer without installing Samba for example, because mplayer was compiled to be able to play files over SMB shares.
It was important to me to have a clean, well maintained, minimial unix system.
Arch looked perfect, but I don't like the fact that it is a rolling release/bleeding edge. Alpine Linux seemed interesting, but it's really more suited to a firewall type device. The rest of the Linux distros don't really meet my requirements and OpenSolaris is dead, which means I had to look to the BSDs.
I will never run OpenBSD, as I don't think they understand what security is, and do a lot of showboating. They are great OpenSSH maintainers though. FreeBSD is just too big, and s hard to get minimal. There is no advantage in running FreeBSD over NetBSD for me.
Enter NetBSD. It seems absolutely perfect. It has package sets like Slackware and a full install still fit's on a CD. It's under 300MB without a desktop environment. It's stable, and only has major releases about once a year. Security updates are pushed out, it can run linux binaries for software you can't compile, and it has some very interesting security features. Specifically kernel level file monitoring for executable and config files, full disk encryption, and a port of PaX which I think is a superior implementation of ESP when contrasted with W^X.
Of note to me also, is the Linux teams attitude towards security. Linus and Greg K-H have gone on record multiple times stating that security issues do not deserve special treatment and do not need to be fixed sooner than other bugs, since they are "just bugs". That is a dangerous attitude, and leads to things like the 'Wunderbar Emporium' exploit allowing root across many major kernel version releases.
The NetBSD team takes security seriously, and issues security updates as a priority as needed.
For anyone truly looking for a secure, minimal and clean *nix system as a replacement for Slackware, I can't recommend NetBSD enough.