There’s a previous thread on this topic here: https://www.reddit.com/r/AskHistorians/comments/1fdv8q/what_happened_to_the_trenches_after_wwi/

I can speak to the fact that not all of them were filled in. In Ypres, for example, you can visit the old trenches.

I finally found an old answer I wrote on this question.

The long and short is that it varies quite a bit on location. But a lot of the parts of the Western Front that were fought over the longest were demarcated as a "Zone Rouge" that had limitations on the permitted human activities allowed there, because of the heavy quantity of unexploded ammunition and toxins like lead, arsenic, chlorine and mercury. These particular areas were essentially fenced off and left to nature.

ETA - of course that's not all areas. Some areas had and have more active human use (which does mean an "iron harvest" of World War I ammunition from farms' fields occurs every year), but also areas of particular significance (like Verdun and Vimy Ridge) were memorialized with cemeteries and monuments.