See https://github.com/languagetool-org/german-pos-dict:

The German data for part-of-speech tagging is taken from Morphy
(http://morphy.wolfganglezius.de, https://danielnaber.de/download/wklassen.pdf) with extensions
and corrections from Julian von Heyl (https://www.korrekturen.de/flexion/)
under Creative Commons Attribution-Share Alike 4.0.
