Thanks guys for your advice. Here an working example with the “new” template sensor. Thermostat is a Meross MTS200.
template:
- sensor:
- name: "Temperatur Wohnzimmer"
unique_id: "template_sensor_temperatur_wohnzimmer"
state_class: measurement
device_class: temperature
unit_of_measurement: '°C'
state: "{{ float(state_attr('climate.thermostat_wohnzimmer', 'current_temperature')) | round(1) }}"