I frequently use single threaded async runtimes in Rust. Particularly if it's background processing that doesn't need to be particularly high throughput.

Eg in a user application you might have the performance sensitive work (eg rendering) which needs to be highly parallel - give it a bunch of threads. However when drawing the UI, handing user input, etc you usually don't need high throughput - use only 1 thread to minimise the impact on the rendering threads

In my work with server side code, I use multiple async runtimes. One runtime is multithreaded and handles all the real traffic. One runtime is singlethreaded and handles management operations such as dispatching metrics and logs or garbage collecting our caches