For the vertical list there is a ready example:: https://mdbootstrap.com/docs/standard/components/list-group/#section-flush

If you want to change this example and apply the solution I gave for llist-inline you can modify the styles:

.list-group-flush > .list-group-item {
    border-width: 0;
}
.list-group-flush > .list-group-item:not(:last-child):after {
    content:" ";
    position: absolute;
    height: 1px;
    bottom: 0;
    width: 100%;
    left: 1rem;
    background: #666666;   
}

The inline-list does not provide such a separator. You can add it yourself by adding some css code, e.g .:

  .list-inline-item:not(:last-child):after {
    content:" ";
    margin: 0 .5rem;
    position: absolute;
    height: 1rem;
    top: 4px;
    bottom: 0;
    width: 1px;
    background: #666666;   
  }