At the moment of this writing, Bootstrap has been removed from Sage 10 in favor of Tailwind CSS.
You could remove Tailwind CSS with yarn remove, remove Tailwind CSS from webpack.mix.js, delete Tailwind CSS configuration file (tailwind.config.js), remove all css & mixins from the SCSS files.
An easier way would be to clone the Sage repository, and checkout the last commit before Bootstrap was removed:
git clone https://github.com/roots/sage project-name git checkout 33005e59
Remove the .git directory.