2015-09-14 UTC
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Zegnat http://www.w3.org/TR/capability-urls/ (as linked by rhiaro in #indiechat) also lists some examples of unguessable URLs, though it seems to be more aimed at URLs to replace passwords and possibly only be temporary, while unguessable URLs like from Google are meant like real URLs, a fixed location for a resource that is meant to be shared.