There is always more to say, but you're referring to what is colloquially known as the "live and let live" system, which certainly was in place, informally, in certain places and at certain times in World War I. I discussed this system in an earlier response and you might like to review that while you are waiting for fresh answers to your query.

In addition, u/backforkippers also writes of 'quiet' sectors and the 'live-and-let-live' system.