The proper question is, when did threads stop being some kind of inscrutable black magic? And the answer is, if you consider the whole of the programming space

The proper question is, when did threads stop being some kind of inscrutable black magic? And the answer is, if you consider the whole of the programming space, that it's still a work in progress. There's a lot of good ideas in various places, but I'm not sure I've seen it unified into one coherent whole in any one language in such a way that it's simply a solved problem for virtually no effort. Every A-list or B-list language I know either has easy concurrency that can still get you deep in trouble (like Go), or "solves" the concurrency problem but at a fairly steep cost to the programmer (like Haskell). Arguably Pony is the closest I've seen, but that's not even B-list yet, that's an "up and comer".