To my knowledge, there are two official(?) Weather plasmoids: the 1x4 in KWeather, and “Weather Report” from kdeplasma-addons. I haven’t been able to build a working KWeather applet for Plasma 6 due to QML errors emitted by Kirigami. And while Weather Report works out of the box, it only uses Data Engine sources, which I’ve personally found to be less reliable than met.no.
Would it be worth implementing KWeatherCore support in the Weather Report plasmoid? I’ve been considering it, but if there are other plans, I’d rather look into that.