If you want to keep using bootstrap you can still utilize the legacy brick @coremedia-examples/brick-bootstrap by creating a copy and add it as a dependency to your theme (please also check the eject feature described in the Frontend Developer Guide). Please note that the brick will be removed in the next AEP.

This enables customers who don't want to utilize the framework to use our bricks while still supporting the framework. As a side note you can now control which version of bootstrap you want to use by adjusting the "bootstrap-sass" dependency and the imports on SASS and JavaScript side accordingly.