Add ‘last’ class to menu item

Have you considered the CSS pseudo class :last-child element?

Here is a great article to help explain: https://css-tricks.com/pseudo-class-selectors/

IE 8 does not support last-child. It would be much easier to add a class to the last li in list.

<li class="menu-item menu-item-type-post_type menu-item-123" id="menu-item-123"><a href="https://www.example.com/about-us/">About</a></li>

<li class="menu-item menu-item-type-post_type menu-item-123 menu-item-last" id="menu-item-123"><a href="https://www.example.com/about-us/">About</a></li>

Also, you can give the optional class for the last menu item from admin.