hugo-theme-introduction/layouts/partials/blog/archive.html

<section>
  {{ $prev := now.Format "2006"}} <!--start from the current year-->
  {{ range first 1 .Pages.ByPublishDate.Reverse }} 
    {{if .Date}} <!--also add the header if there are posts during the current year-->
      {{if eq $prev (.Date.Format "2006")}} <h2 class="title is-2 top-pad"> {{ $prev }}</h2> {{end}}
    {{end}}
  {{end}}

  {{range .Pages.ByPublishDate.Reverse}}
    {{if .Date}}
    {{$curr := .Date.Format "2006"}}
      {{if lt $curr $prev }}
          <h2 class="title is-2 top-pad">{{ $curr }}</h2>
      {{end}}
    <li class="post-item">
      {{ if .Params.date }}
          <span>{{ .Date.Format (.Site.Params.dateFormat | default "Jan 02, 2006") }}</span> -
      {{ end }}
      <span><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></span>
    </li>
      {{ $prev = $curr}}
    {{end}}
  {{end}}   
</section>