1. Cockney rhyming slang. Well, maybe it's unpopular.
2. Privatised railways. "The unpopularity of rail privatisation is an odd phenomenon. ... The story of rail usage under British Rail was one of inexorable decline. Between 1960 and 1995, passenger numbers fell by about a third. Since 1995, they have more than doubled. The dramatic trend reversal coincides exactly with privatisation. ... privatisation has achieved many of the things its proponents hoped it would: better trains, new timetables, more responsiveness to passenger needs." So says John Kay.
3. Gunslingers, now extinct. Here is the TLS, in 1928, in an elegiac mood for the Old West.
4. Quantum Russian roulette. This is "a version of Schrödinger’s cat in which the experimenter stands in for the unfortunate animal. In some futures, he will be killed. In some, he will remain alive. But since, from his point of view, he will be aware only of the latter, he will always perceive that he survives." Actually this might be very popular across the multiverse, just not so much so locally.