Well is there anything still there? Some places still have their city walls, which are still physical walls (albeit not used as such), hence the `barrier=city_wall`. `historic=citywall` is a good tag to add as well (not: `historic` not `historics`).
If the structure has been completely removed, then there is no more wall/barrier there any more, and it should be removed from OSM.OSM.
**Update after edit**:
> A part is still intact, for those the tags make sense, I agree. Other parts are completely gone. Should the historic be kept? On other parts, there is a special pavement indicating that there was a wall in ancient times. What about those?
If there is no physical wall there, then the `barrier=city_wall` should be removed. Because there is no longer any barrier there. If there are/is a monunment to the old wall, you can map that. Otherwise, if the object isn't there, delete it.