One aspect of virtual threads that has gone somewhat unappreciated so far is that if you (ab)use some internal APIs you can probably implement the testing technique used by FoundationDB.

For those who don't know, FoundationDB is a horizontally scalable transactional (KV) database used by Apple for their cloud services. It's famous primarily for its testing approach in which they can run the entire set of microservices in a simulator that can both inject faults, but also completely control the scheduling of work. In this way they make the system entirely deterministic, and can then pseudo-randomize simulations to uncover race conditions and the like.

To do this they created a custom DSL on top of C++ called Flow which is compiled to coroutines.

https://www.youtube.com/watch?v=4fFDFbi3toc

Flow and what it enables is considered (by them at least) to be a key competitive advantage over other database engines.

With Loom the JVM now happens to support the very same thing, just not officially. That's because the JVM makes file and network IO fully "pluggable", virtual threads can be supplied with a custom scheduler, and classloaders can be used to run multiple programs within the same JVM (with some caveats). So if you wanted to make a deterministic simulation of your microservice based system then you could do that now.

The above would be a rather advanced project for intrepid developers only, but it'd be interesting to see someone explore that at some point.

Interesting idea. Are there any implementations of this sort of thing in the wild?

Rust's tokio has a project called loom as well which can do permutation testing of async code: https://github.com/tokio-rs/loom