Just FYI, UBA is now offering a version 3.0.0 of the API, which is partially documented at https://www.umweltbundesamt.de/daten/luft/luftdaten/doc
With some digging there is even a machine readable spec at https://www.umweltbundesamt.de/sites/all/modules/custom/w21_air_data/doc/uba-air-data-api.v3.yaml
However, it is incomplete (e.g. the available CSV endpoints are not included) - so updating and enhancing the spec here could still be valuable.
Just FYI, UBA is now offering a version 3.0.0 of the API, which is partially documented at https://www.umweltbundesamt.de/daten/luft/luftdaten/doc
With some digging there is even a machine readable spec at https://www.umweltbundesamt.de/sites/all/modules/custom/w21_air_data/doc/uba-air-data-api.v3.yaml
However, it is incomplete (e.g. the available CSV endpoints are not included) - so updating and enhancing the spec here could still be valuable.