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Objective C does (explicit) reference counting, though I suppose that's a pretty limited use case these days.

Doesn't Swift (built on ObjC) also use reference counting?

Yes, Swift uses refcounting.

Does Swift have a GIL? If not, how does it solve the problem of multithreaded reference counting?

Per earlier comment[1], Swift uses the biased refcounting approach Gross is proposing for Python.