Goroutines are probably closer to Fibers which D also has:

https://en.wikipedia.org/wiki/Fiber_(computer_science)

They used to be closer to fibers years ago, when they were running on a single thread by default. Now they behave exactly like normal threads with the same problems. And fibers are threads anyway.

Care to share a reference to them no longer being lightweight?

They still start with tiny stacks, if that's what you are asking. I believe it's somewhere around 2 KB nowadays.

goroutines are anything but threads.

Here's a 40min talk explaining how Go's scheduler works:

https://youtu.be/YHRO5WQGh0k

No, they are threads. It doesn't matter if they start with tiny growing stacks and whether blocking syscall wrappers call into the scheduler. Those are just implementation details.

I mean, sure, but they have all the same problems as threads for most / application-level developers. They're indeed quite different when you get into calling out to C or other kinds of FFI, but relatively few developers need to do so directly. For everyone else, they come with very nearly exactly the same semantics as threads.