Simple template sensor with last state memory

Configuration
2

I suggest you use a Trigger-based Template Sensor because its value is preserved after a restart.

template:
  - trigger:
      - platform: state
        entity_id: sensor.sensor_pm10
    sensor:
      - unique_id: sensor_pm10_LastState
        name: "Wrocław PM 10{{ ' (offline)' if trigger.to_state.state == 'unavailable' else '' }}"
        unit_of_measurement: "μg/m³"
        icon: "mdi:molecule"
        state: >
          {% set pm10 = states('sensor.sensor_pm10') %}
          {{ pm10 if trigger.to_state.state != 'unavailable' else this.state }}

Its initial value will be unknown until it is triggered (by sensor.sensor_pm10).

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