您应该为li的最后一个添加样式，即#headerlinksli:last-child,a

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#headerlinks li, a {
    display: inline-flex;
    text-decoration: none;
    color: black;
    border-right: solid 1px black;
}
#headerlinks li:last-child, a {
    border-right: none;
}

 <div id="headerlinks">
        <nav>
            <ul>
                <li><a href="#"> Contact us </a></li> <!-- need to link and create contact us html -->
                <li><a href="#">Accessibility</a></li>
                <li></li><a href="#">Login</a></li>
            </ul>
        </nav>
    </div>

如果您想从所有标记中删除边框，那么只需从css中删除边框-右：固体1px黑色

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#headerlinks li, a {
    display: inline-flex;
    text-decoration: none;
    color: black;
    border-right: solid 1px black;//remove this if border not req.
}

<div id="headerlinks">
        <nav>
            <ul>
                <li><a href="#"> Contact us </a></li> <!-- need to link and create contact us html -->
                <li><a href="#">Accessibility</a></li>
                <li></li><a href="#" style="border:none;">Login</a></li>
            </ul>
        </nav>
    </div>