いろいろ推してきたGitHub Actions、macOS 10.15追加と同時にmacOS 10.14削除かつXcode 9.x~10.x削除と、まったく推せない状態に… https://github.blog/changelog/2019-11-06-github-actions-macos-virtual-environment-updated-to-catalina/

GitHub Actions - macOS virtual environment updated to Catalina - T...

The GitHub Actions macOS virtual environment has been upgraded to Catalina (v10.15). Jobs using the macos-10.14 virtual environment will not run and must be migrated to use macos-latest. In addition to Catalina, other changes include: Xcod...