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Is CPython the only widely-used language implementation that uses reference counting rather than tracing garbage collection?

IIRC even PyPy and MicroPython use tracing GC.

perl5 uses reference counted garbage collection (since perl 5.8 - came out in 2002). They avoid the GIL by having a seperate interpreter instance per thread https://perl.mines-albi.fr/perl5.8.5/5.8.5/sun4-solaris/thre... If you want to share variables between threads, then they have to be marked as shared https://perl.mines-albi.fr/perl5.8.5/5.8.5/sun4-solaris/thre...

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.