Hello Tajtrdlik,
 
is this:  
"This observationtower is closed"  
the only message you have received?
 
Was there maybe a up to date photo of the entrance of the tower send to you too? That would help greatly to decide if the mapping should be changed or not.
 
Because there are less than a hand full of active Mappers in this big area: https://www.openstreetmap.org/relation/1739376 and it is likely that nobody of them will drive to this tower for years just to check if it is closed or not.
 
greeting Peter 