How to check the first item/content on Drupal View
Have you tried to print other stuff of the loop variable, maybe {{ loop.index }}, did this work? You then could if-request this. As far as I remember loop.index worked fine in Drupal.

Next thing you can try is to iterate over the keys and if-request the key for your logic. Pretty much the same as loop.index.

{% for key, items in related_content.entity.field_content_items %}
  {% if key == 0 %}


Last resort would be to target related_content.entity.field_content_items[0] directly.

But I think the first two suggestions should give you at least one opportunity to do what you want.



Thanks for the clarification. Views custom text only. OK, in that case consider to use 2 views (or just 2 displays of the same view) as outlined here https://drupal.stackexchange.com/a/104608. Then you'd attach the first view (or display) - which is limited to display only 1 row - before your second view (there must be an attachment option somewhere) and your second view then should be configured to have an offset of 1.