Its merged into CPython 3.13 but labeled as experimental.

Single threaded cpu bound workloads suffer in benchmarks (vs i/o workloads) till they put back the specializing adaptive interpreter (PEP 659) in 3.14. Docs say a 40% hit now, target is 10% at next release.

C extensions will have to be re-built and ported to support free threaded mode.

Some interesting and impactful bits of open source work for those with a c++ multithreading background.