So, I indeed have historical estimates now, but just to be clear this is not LTS. I donot know how HA works but I guess it fires a action/service to call per date and forecast.solar is not providing historical data (without an API = price)…this one does.
I do see the idea of the OP to be able to finetune forecast settings (inclination/azimuth/etc) but my guess is that this is one-off only where as soon as this is fine, why look back in time?
Why do You think that it is not LTS? In attached image You see that we have 14 days recorder and older data is LTS (data samples period became longer). So IMO it is recording LTS 
Best, JR
Because it is not visible in the db…that is why
EDIT: the only one showing for this integration is: sensor.power_production_now
And this has only been active since yesterday, which does not explain values 10+ days ago
to get LTS storage, a template sensor must be numeric and have an entity_category and a device_class.
Show your sensors under the dev tools state tab. If it has a valid state class, it will be in LTS.
Where did you look? It won’t be in the states table. The LTS tables are separate.