Since there is not an update on this, I decided to do my own.
SINCE THIS CANNOT BE DONE ON A CHILD-THEME, EVERY THEME UPDATE WILL REVERSE THIS

TagDiv, you can easily adapt this into a new update:

/wp-content/themes/Newspaper/includes/wp_booster/td_first_install.php
line 375 add
'xing' => '',

/wp-content/themes/Newspaper/includes/wp_booster/td_social_sharing.php
line 226 add
case 'xing':
return array(
'url' => 'https://www.xing.com/app/user?op=share;url=' . $page_permalink,
'title' => 'Xing'
);
break;

Since I don’t want to rerender all .less files but to get something displayed, you can use this css in your customcss up until tagdiv adapts this into their theme:
/* Adds the XING ICON for Sharing */
.td-icon-xing {
background-color: #005F5E;
width: 40px;
height: 40px;
margin-left: -13px;
padding-top: 14px;
}
.td-author-social > .td-social-icon-wrap > a > i.td-icon-xing {
padding-top: 0;
font-size: 15px;
width: auto;
min-width: 18px;
height: auto;
margin-right: 15px;
color: #fff !important;
}


What TagDiv needs to do:
Generate a new tagDiv.svg Unicode from IcoMoon for XING and put it there,
then:
font-icons.less (mobile and desktop version)
.td-icon-xing{ background-color: #005F5E };