[dataflow]

C++ has had closures without a GC for over a decade now.

[pjmlp]

As syntactic sugar for how C++ functors used to be implemented, and they also bring their own set of issues when interoperating with co-routines.

[dataflow]

If anything that's a statement about coroutines, not closures. And even then, not every coroutine has problems. I don't see a logical implication from that to a GC requirement.