I found there is almost no rust unstable feature usage inside substrate, is that a coincidence, code of conduct, or for some specific reasons?
We only need rust nightly for compiling the runtime and there is also some open issue to switch to stable as soon as we can. The main point here would be that communicating which nightly compiler for every version would be a little bit annoying. We already had these problems when nightly changed stuff that we used in the runtime.
In your own Substrate based runtime you can enable whatever rust feature you like. We will not prevent this or stopping you from doing this :)