As of Home Assistant Core 2024.4 we have deprecated the auxiliary heater functionality in ClimateEntity.
Integrations that are currently implementing the is_aux_heat property and the turn_aux_heat_on/turn_aux_heat_off methods need to remove these and alternatively implement other entities to accommodate the necessary functionality such as a SwitchEntity or in the case of a read-only property a BinarySensorEntity.
You can read more about this decision here.