I was trying to update the rand dependency of the various utilities in https://github.com/uutils/coreutils, and I noticed that ReadRng was deprecated. I don't know the exact reason for this deprecation, the note just says removal due to lack of usage. So, in any case, here's our usage scenario:
We're trying to implement the --random-source argument that many utilities support. This flag allows to specify a file as a random source, and it seems like ReadRng is the easiest way to support this.

It won't be a big problem to work around this by either copy-pasting the deprecated implementation or by not updating the rand dependency, but maybe you can reconsider the decision to deprecate ReadRng.