Mostly they have.
A few years ago, I stayed at a hotel just outside Ypres (in Belgium) which more-or-less stood dead on what was the front line, back in 1914–18. There was a bunker in the garden, and lots of photos of what the immediate area looked like during the war (a muddy, blasted, hellscape).
Next door was a water park, across the road were fields.
If you look at aerial shots of, for example, the Lochnagar crater, which was right on the front line, and which was created by a massive underground explosion on the first day of the Somme battle (July 1916), you’ll see that it’s surrounded by fields of crops.
The areas that haven’t been reclaimed are the ones where the fighting was so intense, that there are still significant amounts of explosives, posion gas, etc, buried in the soil. Just down the road from Lochnagar is the Canadian (well, Newfoundland) memorial, at Beaumont-Hamel. There are areas there that are fenced off, and clearly marked as dangerous, becasue there are still explosives in the ground. Whilst the line of the Newfoundland trenches has been kept intact, and they are still there, along with no-man’s land, and that area has been turned into a memorial, the site of the German front line 100–200 yeards away has been cleared and reclaimed as farmland.
So, most of it has been cleared, but certain areas (the ‘zone rouge’) have not.
Think about that. We were so clever in our destruction of each other that we literally rendered the earth uninhabitable, for over 100 years.