I use soap on my cast iron and I always have.

The argument is that dish soap will break down the oil used to season it. But when the cast iron is seasoned, the oil binds with the iron and becomes polymer and polymer isn’t broken down by soap easily, especially modern soaps that are not nearly as harsh as soap used to be.

Think about cleaning a Glock, which is polymer. Perhaps you’ve heard that one of the best ways to clean a Glock is w/dish soap. Provided it’s dried thoroughly after.

So as long as you have a good solid season and thoroughly dry it right away, you’re ok to use soap on cast iron.