Comments filter by Last in Twig
The last item in comments is the pager, which you have to remove first to get to the last comment. Then you can build a new render array with the single comment and the pre-render callback:

{% set last_comment = comments|without('pager')|last %}
{% set pre_render = comments['#pre_render'] %}
{% set last_comment_build = { '#pre_render': pre_render, '0': last_comment } %}
{{ last_comment_build }}