Incorporating a feature to reduce unused CSS, using Gulp etc., was suggested long ago and a feature request was made some time after that - but it only received two votes.
One of the difficulties in removing unused CSS is handling the cases where classes etc. are created dynamically with Javascript, PHP or even from a data source - ie the issue of adding exceptions. I’ve suggested that Wappler would be well placed to deal with this, given it will provide most of the relevant Javascript in many cases. It would be good to see something implemented - whether for Bootstrap and perhaps other frameworks in the future. However there will still be cases where using Purge CSS etc. will break things.