All time maximum value for a sensor

Configuration
5

You might consider a trigger-based template sensor. It can’t have its value overwritten from the UI, and it combines an automation and an entity into one thing.

template:
  - trigger:
      - platform: state
        entity_id: sensor.openweathermap_temperature
        not_to:
          - unavailable
          - unknown
    sensor:
      - name: Highest Temp Ever
        unique_id: c6174f4d-0709-4e70-a675-5ba3f07c80a0
        state: >
          {{ max(trigger.to_state.state | float, this.state | float(-999)) }}
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