Starting with Bootstrap 4, the '.col-xs-*' class is no longer used. For reference, see the official blog post:
Back in Bootstrap 3, .col-xs-* was how you handled layouts for extra small screens. But with Bootstrap 4 and up, things got simpler thanks to a mobile-first approach.
You don’t need a special class for extra small devices anymore. Just use .col-* without any breakpoint prefix—it automatically applies to screens smaller than 576px.
If you're working with different layouts for various screen sizes, you can still use classes like .col-sm-*, .col-md-*, and so on for small, medium, and larger devices. For extra small (XS) screens, just stick with .col-* and set the value you need.
For more details, refer to the Bootstrap 4 Grid documentation: