It never was in Slackware, as well as it never was in OpenBSD. And in the latter, it probably never will, with no way to install it by yourself.

As for me, this is enough to don't use PAM in my programs. Unless I would think it's worth implementing both PAM and PAMless versions (which I never did).

On the other hand, PAM can be added to Slackware and all other Linux distros. So when you're not thinking about OpenBSD (other UN*Xes have PAM support), you can just add PAM to the dependencies list.