I am no longer using Agenda and have removed it from all my devices. (Sorry)
If I manage iCloud data I can see that Agenda is taking up some space that is no longer needed. The usual process is to select Agenda, and then choose the option to delete all data. This does not work for Agenda and the item remains in the list of applications storing data in iCloud.
I have successfully removed other applications from this interface so would appear to be a bug in Agenda only.
I have attempted to remove this data using devices on iOS 13 and 14, and also from a Mac running macOS 10.15.6