It is still very relevant as an extension language for C (and C-likes, I suppose).

If you have all these constraints:

- must be easy to use extensions languages functions from C, and the other way around

- must therefore be easy to convert values from the extension language to C, and the other way around

- must be possible to share this state in a multi thread program

- including, have several threads implemented in the extension language

- must be able to load code at runtime

Then guile is not only a good choice, it's sadly the only possible choice (or at least that was the case last time I checked few years ago). Many languages that claim to be good at extending C are lacking one of the above (usually the multithreading part; Lua, for instance, allows to start several VM in several threads, but they are isolated).