So... Yesterday, Jon Arndt found an obscure bug over on Super User:
Normally, when looking at tag information, at the bottom of the screen, there is a button labeled "Edit Tag Info".
However, when looking at [the slackware tag] today, there is no way to suggest edits.
Turns out what happened here was that the original wiki was suggested by a user who was later destroyed for engaging in nefarious behavior; in these situations all posts owned by the user are deleted along with them - including, as it turns out, tag wikis or tag excerpts (the first editor gets implicit ownership of the "post" that backs these wikis).
This gets even more annoying because there's no way to undelete a tag wiki - every undelete route has a check to verify that the post being undeleted is either a question or answer before proceeding.
Eventually, I landed on a solution: create a new tag, merge the broken tag into it (thus orphaning the deleted wiki), and then rename the new tag to be the same as the old one (thus keeping all the tagged questions but introducing a new wiki and excerpt).
There were only 4 cases of this network-wide, so I went ahead and just applied this goldbergian fix to all of them... Forgetting in the process that support is a special tag: internally, there's a flag (set on the creation of the site) that marks it as fulfilling the "required" requirement on meta. Of course, this was lost in the merge...
Geoff Dalgas has helpfully reapplied the flag, so support should be back to normal again after the next build. Sorry for the confusion!