I can name a few. For example Neukölln in Berlin, or some areas in Köln (like Kalk, Ossendorf, Chorweiler, Porz), or the north of Duisburg. You can find lots of references online - search for "no-go zone germany", or simply befriend a German policeman.
All these are areas where policemen are afraid to work because they may get suddenly surrounded by 40-50 people. It's true, you can still access them, but then it doesn't mean you get out easily, hence they are not accessible.
http://www.spiegel.de/panorama/justiz/nordrhein-westfalen-po...
https://www.derwesten.de/politik/in-problemvierteln-fuerchte...
All these are areas where policemen are afraid to work because they may get suddenly surrounded by 40-50 people. It's true, you can still access them, but then it doesn't mean you get out easily, hence they are not accessible.