esphome:
  name: set-internal-at-boot
  on_boot:
    then:
      - lambda: |-
          id(hidden_at_boot).set_internal(true);
          id(shown_at_boot).set_internal(false);

host:

api:
  actions:
    - action: set_internal_late
      then:
        - lambda: id(untouched).set_internal(true);

logger:

sensor:
  - platform: template
    name: "Hidden At Boot"
    id: hidden_at_boot
    lambda: return 1.0;

  - platform: template
    name: "Shown At Boot"
    id: shown_at_boot
    internal: true
    lambda: return 2.0;

  - platform: template
    name: "Untouched"
    id: untouched
    lambda: return 3.0;