Bootstrap col-xs-* not working?

Question

Bootstrap

Why don't col-xs-6 work on phone screens, all blocks one after another,

<div class="container">
    <div class="row">
        <div class=col-xs-6 col-sm-4 col-md-3 col-lg-2 col-m">Block 1 1</div>
        <div class="col-xs-6 col-sm-4 col-md-3 col-lg-2  col-m">Block 1 2</div>
        <div class="col-xs-6 col-sm-4 col-md-3  col-lg-2 col-m">Block 1 3</div>
        <div class="col-xs-6 col-sm-4 col-md-3 col-lg-2  col-m">Block 1 4</div>
        <div class="col-xs-6 col-sm-4 col-md-3 col-lg-2  col-m">Block 1 4</div>
        <div class="col-xs-6 col-sm-4 col-md-3 col-lg-2  col-m">Block 1 4</div>
    </div>
</div>

But if you add "col-6" Block 1 1 it will work, then why do you need col-xs? Yes, even the examples from the official site bootstrap do not work

Answer 1

Bootstrap 4 doesn't have col-xs-* ( Proof )

Answer 2

col-xs-* was before version 4, from version 4 col-*

Answer 3

bootstrap4
https://v4-alpha.getbootstrap.com/layout/grid/#gri...