For cleaning cast iron, I rarely need to use soap. If there's some stuck on gunk that cant be cleaned with water and a sponge, I put it back on the burner and fill it 1/4 full with water to deglaze and steam the gunk off and then wash with water and a sponge.
If there's burnt on stuff, I dump a bunch of kosher salt and oil into the pan, cut a potato in half and buff out burnt bits away.
Using soap is fine. If your pan is properly seasoned, it cannot wash away the polymerized oil.
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