Simplifies complex Jinja templating logic which is brittle and breaks across Ansible versions.

This is a common enough pattern I think a custom filter makes sense. There is another alternative to avoid the for loops but it's quite complex:

{{ wordpress_sites | dict2items | selectattr('value.env', 'defined') | selectattr('value.env.db_host',
  'defined') | selectattr('value.env.db_host', 'ne', 'localhost') | map(attribute='key') | list }}