[int_19h]

It's not enough to pass arbitrary C++ classes, either. The point is that you can still design a fairly expressive API involving structured data within the constraints that the C ABI imposes on you - and that C ABI will guarantee stability. This is how plugins were usually handled in the C++ world, historically speaking.

(There are also higher-level ABIs like COM, but they are usually reducible to the C ABI in practice - e.g. COM can be described entirely in terms of struct and function pointers.)

[yencabulator]

You can write C plugins, or C-ABI plugins in any language that can do that, and call them from Go just fine. Nobody just considers that a very good way to write Go, or plugins for Go.