If the script:

- command: "whoami"
  become: true
  become_user: "{{ deploy_user }}"
  register: u
    - f

- debug: msg="{{ u.stdout }}"
    - f

Produces the error:

Failed to set permissions on the temporary files Ansible needs to create when becoming an unprivileged user

Install the acl package that provides setfacl so that proper permissions can be applied to the /tmp folders. [Source]

- name: Install acl
    state: latest
    update_cache: true
      - acl

Ansible documentation is broken

Many links from Google and Ansible's own error messages from versions past are now broken. It's unfortunate that RedHat did not care enough to redirect pages to their new "improved" module doc hierarchy.