I've been using slack for a couple of years now, and haven't ever had to use an initrd. I've used ext2, ext3, and now reiserfs on my root partition without the need for it.
All of the packaged slack kernels should include support for the filesystems available during install.
Also, since a default full install of slack doesn't include an initrd, I wouldn't figure that a kernel upgrade would require one. That would get confusing.