Vendor prefixes are not used inside SCSS instead we use Autoprefixer to handle intended browser support via CSS prefixes. After compilation CSS styles are passed through Autoprefixer to add necessary vendor prefixes. As you know vendor prefixes are important to ensure cross-browser compatibility of latest CSS3 features.
Autoprefixer is an integral part of the project building process. To customize its settings, you should navigate to the
.browserslistrc file. It is currently configured with Bootstrap's default list of browsers.
Generally, it's a good practice to collect some analytics about what Browsers/OSs your visitors use. And then customize Autoprefixer settings based on this data.
For more information about available Browserlist options visit https://github.com/browserslist/browserslist.