The readme does mention the specifics, immediately after mentioning nightly.

> BREAKPOINTS REQUIRE ENABLING THE EXPERIMENTAL core_intrinsics FEATURE

How do I know which versions of nightly support that feature, and that specific version of the feature though?

I like your username.

Just use a recent nightly and you should be fine. Rust project doesn't offically provide support even for old stable versions, so "using nightly" with no specifics generally means using any nightly build, around or newer than, the current stable release.