Offi Plans was apparently removed from the Playstore one more time, apparently because there was a “donate button”. It’s now back on Google Play Store.
A link to an article in German: https://www.heise.de/newsticker/meldung/Nahverkehrs-App-Oeffi-ist-zurueck-im-Play-Store-4153880.html
A lot of FLOSS apps have a donate button, even if they are on the Play Store. There is a risk for them be removed without any warning.