Arcane is an adjective that describes someone or something as being less known or hard to understand except for a few people. Arcane things are hidden or secretive. Obscure is another good synonym. This is not always a good thing.
The adverb form is arcanely.
Obsolete is an adjective that describes something or someone as being out of use because of a new version or more popular item. It can also mean something that is not current or old-fashioned.
The adverb form is obsoletely, though this is seen more often as a misspelling for absolutely than anything else. And the noun form is either obsoleteness or obsoletism.
Avago Technologies Ltd.’s pending takeover of Broadcom Corp. taps an arcane tax structure that’s being dusted off amid a rise in cross-border mergers. [The Wall Street Journal]
Kalman and Horn’s habit of letting their unmoored heroes blend together, save for their arcanely singular fields of study, can make this feel like an ironic riff on Terrence Malick’s The Thin Red Line, where a very different sort of sprawling cast similarly shares one stilted language. [Torontoist]
Over 120 obsolete laws have been repealed by the Narendra Modi government, while another 945 which have lost relevance are awaiting Parliamentary approval to lose their place in statute books. [The Economic Times India]
From then on, federal operations will continue thanks to the use of the now-customary (and obsoletely named) “extraordinary measures.” [Slate]
We are conditioned by our contexts, consuming yet complaining; discarding gadgets claiming technological obsoleteness; buying what we can live without; criticising the current affairs knowing well how we are a party to it and publicly grumbling without private initiatives. [The Hindu]