(Hartzog 2013) Practical obscurity is a concept which pre-dates the online world. It refers to the impediments to data retrieval. The concept was articulated by the Supreme Court in U.S. Department of Justice v. Reporters Committee for Freedom of the Press. In evaluating the privacy of a “rap sheet” containing aggregated public records, the Supreme Court found a privacy interest in information that was technically available to the public, but could only be found by spending a burdensome and unrealistic amount of time and effort in obtaining it. The information was considered practically obscure because of the extremely high cost and low likelihood of the information being compiled by the public.
I have an example from the UK, which I first learnt of some years ago in connection with the law of defamation and contempt of court:
In the Scottish case of Her Majesty’s Advocate v. William Frederick Ian Beggs (High Court of Justiciary 2001) (No2) (2002 S.L.T. 139). the judge ruled that information held on the internet archives of newspapers was published anew each time someone accessed it. (The Defamation Act 2013 has changed the law on this issue). However, he didn’t take the same view of the paper archives held by public libraries. This distinction takes into account the ease with which material on the internet can be accessed.
The matters were considered again for an appeal case involving the same parties  HCJAC 27 where the judgment makes a different point about online obscurity, saying “It appears also to have been accepted by both sides that those materials were archived material which had originally been published before the criminal proceedings became active on 21 December 1999 and that the action of entering the appellant’s name into a standard search engine would not lead the searcher to these materials. Instead the searcher would have to go to the website of a particular newspaper or broadcaster and then search its archived material” http://www.scotcourts.gov.uk/search-judgments/judgment?id=9c9f8aa6-8980-69d2-b500-ff0000d74aa7
HARTZOG, W. and STUTZMAN, F., 2013. The case for online obscurity. California Law Review, 101(1), pp. 1-49.