variable.update_sensor is an action that is used to change the value of one of the sensor entities that you need to create either through the UI or in YAML.
When I said you have to store the value in an entity, I really meant it. There are probably hundreds of ways to do that (both natively and with custom integrations), but they will all end up with a value stored in an entity.
You can exclude an entity from your recorder database with these settings.
Probably the most flexible way to store variables natively is with the trigger-based template sensor described in this post: