Oh this really is a feature and not a bug. This post explains how to get rid of the permanent message.
Hide persistent notification and icon in status bar
After you have enabled Threema Push, a notification entitled “Threema Push” along with an icon in the status bar will appear. To remove both, disable the “Threema Push” notification channel: Navigate to Android’s notification settings for Threema (e.g., “Settings > Apps > Threema > Notifications”), and turn off “Threema Push.”
For SFOS you can find the settings at
Settings > Apps > Threema > Android(TM)-Settings > Notifications
This can be closed.