https://github.com/ansible/ansible/pull/73190

hosts

[foo]
foo1 fail=False
foo2 fail=True
foo3 fail=False

playbook-failure.log

PLAY [all] *********************************************************************

TASK [Print ansible_play_batch (before failure)] *******************************
ok: [foo1] => {
    "ansible_play_batch": [
        "foo1",
        "foo2"
    ]
}
ok: [foo2] => {
    "ansible_play_batch": [
        "foo1",
        "foo2"
    ]
}

TASK [Print ansible_play_hosts (before failure)] *******************************
ok: [foo1] => {
    "ansible_play_hosts": [
        "foo1",
        "foo2",
        "foo3"
    ]
}
ok: [foo2] => {
    "ansible_play_hosts": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (before failure)] ***************************
ok: [foo1] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}
ok: [foo2] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Simulate failure] ********************************************************
skipping: [foo1]
fatal: [foo2]: FAILED! => {"changed": false, "msg": "fail!"}

TASK [Print ansible_play_batch (after failure)] ********************************
ok: [foo1] => {
    "ansible_play_batch": [
        "foo1"
    ]
}

TASK [Print ansible_play_hosts (after failure)] ********************************
ok: [foo1] => {
    "ansible_play_hosts": [
        "foo1",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (after failure)] ****************************
ok: [foo1] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

PLAY [all] *********************************************************************

TASK [Print ansible_play_batch (before failure)] *******************************
ok: [foo3] => {
    "ansible_play_batch": [
        "foo3"
    ]
}

TASK [Print ansible_play_hosts (before failure)] *******************************
ok: [foo3] => {
    "ansible_play_hosts": [
        "foo1",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (before failure)] ***************************
ok: [foo3] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Simulate failure] ********************************************************
skipping: [foo3]

TASK [Print ansible_play_batch (after failure)] ********************************
ok: [foo3] => {
    "ansible_play_batch": [
        "foo3"
    ]
}

TASK [Print ansible_play_hosts (after failure)] ********************************
ok: [foo3] => {
    "ansible_play_hosts": [
        "foo1",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (after failure)] ****************************
ok: [foo3] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

PLAY RECAP *********************************************************************
foo1                       : ok=6    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
foo2                       : ok=3    changed=0    unreachable=0    failed=1    skipped=0    rescued=0    ignored=0   
foo3                       : ok=6    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   

playbook-failure.yml

- hosts: all
  gather_facts: no
  serial: 2
  tasks:
    - name: Print ansible_play_batch (before failure)
      debug:
        var: ansible_play_batch

    - name: Print ansible_play_hosts (before failure)
      debug:
        var: ansible_play_hosts

    - name: Print ansible_play_hosts_all (before failure)
      debug:
        var: ansible_play_hosts_all

    - name: Simulate failure
      fail:
        msg: "fail!"
      when: fail

    - name: Print ansible_play_batch (after failure)
      debug:
        var: ansible_play_batch

    - name: Print ansible_play_hosts (after failure)
      debug:
        var: ansible_play_hosts

    - name: Print ansible_play_hosts_all (after failure)
      debug:
        var: ansible_play_hosts_all

playbook-unreachable.log

PLAY [all] *********************************************************************

TASK [Print ansible_play_batch (before getting unreachable)] *******************
ok: [foo1] => {
    "ansible_play_batch": [
        "foo1",
        "foo2"
    ]
}
ok: [foo2] => {
    "ansible_play_batch": [
        "foo1",
        "foo2"
    ]
}

TASK [Print ansible_play_hosts (before getting unreachable)] *******************
ok: [foo1] => {
    "ansible_play_hosts": [
        "foo1",
        "foo2",
        "foo3"
    ]
}
ok: [foo2] => {
    "ansible_play_hosts": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (before getting unreachable)] ***************
ok: [foo1] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}
ok: [foo2] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Simulate unreachability] *************************************************
skipping: [foo1]
fatal: [foo2]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: ssh: Could not resolve hostname foo2: nodename nor servname provided, or not known", "unreachable": true}

TASK [Print ansible_play_batch (after getting unreachable)] ********************
ok: [foo1] => {
    "ansible_play_batch": [
        "foo1"
    ]
}

TASK [Print ansible_play_hosts (after getting unreachable)] ********************
ok: [foo1] => {
    "ansible_play_hosts": [
        "foo1",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (after getting unreachable)] ****************
ok: [foo1] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

PLAY [all] *********************************************************************

TASK [Print ansible_play_batch (before getting unreachable)] *******************
ok: [foo3] => {
    "ansible_play_batch": [
        "foo3"
    ]
}

TASK [Print ansible_play_hosts (before getting unreachable)] *******************
ok: [foo3] => {
    "ansible_play_hosts": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (before getting unreachable)] ***************
ok: [foo3] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Simulate unreachability] *************************************************
skipping: [foo3]

TASK [Print ansible_play_batch (after getting unreachable)] ********************
ok: [foo3] => {
    "ansible_play_batch": [
        "foo3"
    ]
}

TASK [Print ansible_play_hosts (after getting unreachable)] ********************
ok: [foo3] => {
    "ansible_play_hosts": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

TASK [Print ansible_play_hosts_all (after getting unreachable)] ****************
ok: [foo3] => {
    "ansible_play_hosts_all": [
        "foo1",
        "foo2",
        "foo3"
    ]
}

PLAY RECAP *********************************************************************
foo1                       : ok=6    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
foo2                       : ok=3    changed=0    unreachable=1    failed=0    skipped=0    rescued=0    ignored=0   
foo3                       : ok=6    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   

playbook-unreachable.yml

- hosts: all
  gather_facts: no
  serial: 2
  tasks:
    - name: Print ansible_play_batch (before getting unreachable)
      debug:
        var: ansible_play_batch

    - name: Print ansible_play_hosts (before getting unreachable)
      debug:
        var: ansible_play_hosts

    - name: Print ansible_play_hosts_all (before getting unreachable)
      debug:
        var: ansible_play_hosts_all

    - name: Simulate unreachability
      command: /bin/true
      when: fail

    - name: Print ansible_play_batch (after getting unreachable)
      debug:
        var: ansible_play_batch

    - name: Print ansible_play_hosts (after getting unreachable)
      debug:
        var: ansible_play_hosts

    - name: Print ansible_play_hosts_all (after getting unreachable)
      debug:
        var: ansible_play_hosts_all

$ ansible --version
ansible 2.10.3
  config file = None
  configured module search path = ['/Users/koki/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.8/site-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.8.5 (default, Jul 21 2020, 10:48:26) [Clang 11.0.3 (clang-1103.0.32.62)]
$ ansible-playbook -i hosts -b playbook-failure.yml > playbook-failure.log
$ ansible-playbook -i hosts -b playbook-unreachable.yml > playbook-unreachable.log