I have found a temporary workaround in regards to Netatmo stations not visible on WU anymore. The weather service MeteoWare Plus can be used to send the Netatmo weather data to public weather networks like “Weather Underground” or “wetter.com”. Please refer to MeteoWare Plus for your Netatmo Weather Station. There are no extra costs for this service as long as there are no deeper weather analytics requested from Meteoware.
However, the Netatmo data will be send only every 20 minutes to WU. From what I have seen this delay of 20 minutes is still sufficient for the Zimmerman method and will not cause any issues for controlling the OpenSprinkler by weather data.
From WU officials it is known that for the time being, existing free WU keys will continue to work unless otherwise noted. I know that using the Meteoware service is no final solution, but it may be some help for OpenSprinkler users that also own a Netatmo weather station and have a free Wunderground API key.