unprompted/_includes/sidebar.njk

<aside class="sidebar">
  <nav class="sidebar-nav">
    <h2>archive</h2>
    {%- set currentYear = null -%}
    {%- for column in collections.columns %}
      {%- set colYear = column.date | year -%}
      {%- if colYear != currentYear %}
        {%- if currentYear != null %}
        </ul>
        {%- endif %}
        {%- set currentYear = colYear %}
        <h3 class="year-label">{{ colYear }}</h3>
        <ul>
      {%- endif %}
          <li><a href="{{ column.url }}">{{ column.data.title | inlineMarkdown | safe }}</a></li>
    {%- endfor %}
    {%- if currentYear != null %}
    </ul>
    {%- endif %}
  </nav>
</aside>