e.g. http://www.smogalarm.org/de/station/DEHE001/ has smog level backlog calendars for 2016 & 2017, but not this year. It shows data for current days, though

The website states

We are sorry to announce that we stopped to update the data because of the last UBA website update which removes the API we were using.

Have you asked UBA to bring the API back, what did they answer? How can we urge them to do so, e.g. by having a lot of opendata people ask them for it?

Smogalarm.org has not provided data from the monitoring stations since 29 September 2019. I don't know if it's due to the server-side installation on smogalarm.org or whether the code needs some changes. If it is the latter, I could take a look at it.

Org

For non experts who want to see if it is safe to breathe outside, the µg/m3 values are quite meaningless and scarily technical. A green-orange-red scale would be helpful. Red for values that are considered hazardous even for one day (hour?), orange if bad on a daily basis, green for safe values.

Perhaps necessary too: show how the combination of PM10 and NO2 values affects health.

I would subscribe to alerts if there would be an indication to how to unsubscribe.

When served over plain HTTP sites cannot request Geolocation anymore.

Please roll out HTTPS.

[Deprecation] getCurrentPosition() and watchPosition() no longer work on insecure origins. To use this feature, you should consider switching your application to a secure origin, such as HTTPS. See https://goo.gl/rStTGz for more details.
searchWhereIam @ f2560c74c947.js:4206
fn @ VM105:4
b @ f2560c74c947.js:761
e @ f2560c74c947.js:766
$eval @ f2560c74c947.js:761
$apply @ f2560c74c947.js:761
(anonymous) @ f2560c74c947.js:766
dispatch @ f2560c74c947.js:66
r.handle @ f2560c74c947.js:66

Feeds are called 'latest alerts' making it hard to find the correct feed in an RSS reader. Adding the station name would be helpful.