How do you flag a resource which is not available anymore?

Hi vocabulers

We have more and more records in LOV of which URIs are 404, unfortunately,
with no replacing resource whatsoever.
See e.g., http://lov.okfn.org/dataset/lov/details/vocabulary_dir.html etc
We want to keep the record in LOV, along with backup versions, such as
http://lov.okfn.org/dataset/lov/agg/archives/dir_dir/file_dir_2006-06-27.n3

We want to flag the URI some way, such as some "offlineSince" or
"validUntil" property, with value a xsd:date. This property would be added
to the VOAF vocabulary, unless someone knows about an existing property to
express that. There are various "valid" properties in DC terms and other
vocabularies, but not sure they capture the expected semantics.

Thanks for any suggestion.

-- 

*Bernard Vatant*
Vocabularies & Data Engineering
Tel :  + 33 (0)9 71 48 84 59
Skype : bernard.vatant
http://google.com/+BernardVatant
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