slack.css

.p-channel_sidebar__badge {
    display: none
}

.p-channel_sidebar__banner {
    display: none
}

.p-channel_sidebar__banner--mentions {
    display: none
}

/* hide the entire sidebar */
.p-channel_sidebar__static_list {
    display: none
}

/* hide the names of the sidebars
I prefer this over the top
*/
.p-channel_sidebar__name {
    display: none
}

/* hide the "unread mentions" red banner */
.c-button-unstyled.p-channel_sidebar__banner.p-channel_sidebar__banner--mentions {
    display: none
}

.c-button-unstyled.p-channel_sidebar__banner.p-channel_sidebar__banner--unreads {
    display: none
}