As much as I hate AMP pages I have been forced to use it due to SEO reasons and bootstrap 4 sites aren't AMP comptabible as yet. They exceed the 50kb css limit by AMP.
There have been various attemps(1) to do it but nothing more be more amazing than to have in-built support for it.
Even if the AMP project is misguided mess the 50kb limit on css is still a good goal for mobile sites.