Including ensemble forecasts into Open-Meteo is already on my backlog. And yes, this will return those 90% confidence intervals.

Forecast variability is difficult to express. For example precipitation probability describes how likely rain is given the forecasted atmospheric conditions. Instead some people assume it is raining in 10% of an area.

Making use of probabilities it also tricky. If you run an ice cream shop, with high operation cost and there is a 50% probability of rain, do you open with the chance of barely anyone coming or keep closed to save cost?

Ensemble forecasts also take up a lot of disk space, because each model ensemble is stored individually.