How to delete an entity that is neither in a config file, nor in an integration?

[123 (Taras)]

Good catch.

It was the reason why I originally asked if you recalled which integration you had used. If it was MQTT then I would have directed you to this post:

Delete (MQTT) entities?

Were the old entities manually defined (perhaps in your configuration.yaml file) or were they automatically discovered? If they were manually defined, delete them from the configuration file. If they were automatically discovered, purge their MQTT Discovery topic. Use an MQTT client, like MQTT Explorer, to determine if there is a topic on your broker like homeassistant/sensor/name_of_your_sensor/config. If there is, use MQTT Explorer to delete it.