How to maintain the last value of an entity when become unknown

[jackjourneyman (Jack)]

Do you mean here?

Strategy for creating last value 'cache' versions of sensors in case the sensor becomes unavailable

I have automations that trigger when time is equal to a timestamp-class sensor. In this case, it’s a sensor managed by an integration that tells me the start time of my free power for the day. sensor.hour_of_free_power_start The sensor occasionally becomes unavailable, like when the integration wants me to log back in. If the sensor is unavailable at the time the automation would otherwise trigger, the automation doesn’t trigger. No surprises there. The sensor is a device_class: timestamp and …

I haven’t read the thread closely, but they seem to be talking about general solutions for large numbers of sensors. Not sure this is a good idea.

For specific sensors, I personally use the custom integration Variables+history:

GitHub - enkama/hass-variables: Home Assistant variables component