Boostrap 4 spacing for mobile

[Arturo58]

Bootstrap has spacing classes such as m-sm-1 and m-md-1. It seems that it has for mobiles also as its written in its documentation: Spacing utilities that apply to all breakpoints, from xs to xl.

However, m-xs-* does not work.

I could not find such a class in its source-code too

Does bootstrap 4 have spacing utility classes for mobiles? How can I apply mt-xs-2 on a button ?