It seems that macos-15 runner is not GA yet. Do you know any official documentation mentioning macos-15 runner?

github/roadmap#986

I found the official resource: https://github.com/actions/runner-images?tab=readme-ov-file#available-images

This PR sounds good to be merged.

I didn't add macos-15-xl as it's unknown whether it's supported or not.

It is not documented in the above document. So let's not include it for now.

It seems that macos-15 runner is not GA yet.

Yes. It's just in public beta.
https://github.blog/changelog/2024-09-25-actions-new-images-and-ubuntu-latest-changes/

(This info was also mentioned in commit message.)

I didn't add macos-15-xl as it's unknown whether it's supported or not.

It is not documented in the above document. So let's not include it for now.

Actually none of the macos-<n>-xl labels are listed in https://github.com/actions/runner-images?tab=readme-ov-file#available-images.

"Good news" is that it's announced that macos-12-xl, macos-13-xl, and macos-13-xl-arm64 will be deprecated on Dec 3, 2024.
https://github.blog/changelog/2024-08-19-notice-of-upcoming-deprecations-and-breaking-changes-in-github-actions-runners/