I think the slash was added in some cases because there's an old German sign with a black arrow on a white background with a red ring, that means _you must go this way_ instead of _you must not go this way_ as we'd read it today. So the slash on all such signs (no left turn etc.) is to make it doubly clear this is a prohibition not an instruction.

Fun fact: the German Empire had an additional convention that if the symbol (e.g. a motor car) was shown in outline instead of filled in, then the prohibition only applied on Sundays.

EDIT: looking at https://commons.m.wikimedia.org/wiki/Category:Diagrams_of_hi..., the 1927 German sign for "parking _allowed_" was a black P on a white background with a red circle; the prohibition sign contained the text "Parken verboten" (parking forbidden). So I can see why the German delegate might have objected to making the new "no parking" sign their old "yes you can park here" sign!