The difficulty here is to distinguish between a wiki which has gone offline forever, and one which is having temporary hosting problems. In the first case I would've thought keeping an entry with 'Inactive' is a bit pointless. Might as well delete the page. In the second case, we lose all the category information by switching all the them to Inactive, only to find the wiki comes back online again a few days later.
I'll propose a different approach: If a wiki seems to have gone offline (perhaps forever), we wrap the categories in a <nowiki> tag or a <pre> tag. If you do this for example:
...the category is disactivated, but the info is still there. Looks a bit ugly, but if the wiki stays offline, we might aswell delete the whole page anyway no? -- Harry Wood 13:26, 30 Jan 2006 (EST)