Unable to answer "What’s the topic of this information board?" – Information Board – https://osm.org/node/12809220881 via StreetComplete 62.2:

Ein Sammelsurium:
Ehrenbreitstein 
Rheinradweg
Festungsbaukunst 
Allgemeines

Attached photo(s):
https://streetcomplete.app/p/326092.jpg