This seems to be a Debian problem. Why is this discussed in the Slackware forum?
Installing Slackware does not touch your debian partitions, let alone a single file on its filesystem.

2 members found this post helpful.

I agree. Maybe this thread should be moved to the Debian sub-forum? Most of the people posting in this thread run Slackware, not Debian.

Maybe it is little more complex problem. If this partition is shared between both Debian an Slackware, then may appear inconsistencies: uid of owner, which can be different on both of systems. My suspicion is that Debian records mounts. But the last mount was made not by Debian. It will give us the clue if OP would post layout of partitions. Because mount from Slackware is differently linked than that from Debian - depending on layout and what is in /etc/fstab, what is in PATH, it is possible that Debian is try to use shared libraries coming with Slackware. Crazy but quite possible.

A (root) partition shared between Debian and Slackware is impossible. The OP mentions that a link to a system file disappeared. If he installed Slackware in his Debian root partition then that would wreak havoc, not just remove a single symlink.

The OP still has not shared the content of his fstab file, and unless he provides this and other background information there's nothing anyone can do for him.
And again, this is not a Slackware related issue. It does not belong here. The issue is with his Debian install.

It is difficult to discuss without necessary information. In my experience managing shared file system between tow distribution is complex task. This why I use ntfs-3g file system for shared partition. Also I try to isolate systems - say never user auto mounts of other system root directory. Layout is important cause by the first post I guess there are at least two hard drives. There is possibility / and /usr are mount points for different partitions. So shortly: we need more information.

Its at least two files, the symlink and the real file the symlink points to. I suspect there could be more missing files the OP hasn't noticed yet, but if he did install Slackware over Debian I would be surprised if Debian would even boot...