The issue is settled now. Germany paid huge war reparations after WWII until the 1990s.
Apart from that, many of these explosives are malfunctioning bombs that were supposed to go off when firefighters and rescue personell are at the site. Such devices are pure malice; the defense argument can't justify them.
You can't randomly levy all sorts of minor nuisance onto somebody after they've conceded and signed a bunch of treaties largely drafted by the winning side meant to settle this. That's just a great way to self-sabotage your diplomatic reputation.
But there is (or rather was) an greater argument, of not wasting anymore time, with historic fights over and over again and rather move on and look forward.
All the great conflicts are getting solved that way and not with bean counting.
Cologne has a found WW2 bomb every other week or so, e.g. look at this non-exhaustive press release list: https://www.stadt-koeln.de/basisdienste/suche/?keywords=Bomb...
Most can be disarmed, but some have to be exploded in a controlled manner.
Nowadays I get pushed safety notifications to my phone (KATWARN app) wherever I am in Germany that they found a WW2 bomb nearby.
The app then shows the location and evacuation information.