Soap does no ruin cast iron!! It's a myth!! https://twitter.com/seungminkim/status/1317831944235814912

Everyone stop saying it ruins the seasoning that's the myth. Polymerized oil isn't going to wash off with soap.

Polymerized oil is like a hard plastic coating over the top of your pan it takes a huge amount of soaking and scrubbing for a very long time to remove it and sometimes even that doesn't work. This is just chemistry.

Have you ever had a discolored sheet pan? That has a super hard brown coating on it that just won't come off no matter what you do?

That's polymerized oil. That's your cast iron seasoning! https://twitter.com/Dogshouse2000/status/1317889114784501766?s=20

(And this BTW is also why you don't have to bother trying to get that stuff off of your sheet pans because it's harmless seasoning, not dirt, and it will just come back anyway.)