Many of the old texts refer to soap, or "soaping up". This appears usually in the context of setting creases. For example in Delafera's The Art of Garment Making it's used in reference to creasing the quilting lines in silk facings of smoking jackets.
I have struggled to find much contemporary discussion of tailors soap. It is sold by The Lining Company, and the Caffeinated Tailor on YouTube uses soap (a generic brand Lagarto) when making up waistbands. Is this something others have experimented with and is it still relevant in today's tailoring?
My first intro to soap on a crease was in the Army Reserve. I used actual soap "imperial leather". But that's what it's for, holding a crease for longer.