I suppose it can be crawled like any other website, since it's a collection of links - manx-docs doesn't host any of the manuals.

As such it can be bitten by the usual link rot problem, but at least it has multiple links for each entry, which should minimize that. Where no digitized copies are known to exist, the entry is listed without links - but that's still a good thing to have, for those with the means to do something about it.