That'sWinston and Walter, a Canadian company, does. I saw it on a YouTube channel.
The link below is just one product. They appear to do it on most of their molds, including slabs. They appear to mostly do tall and skinny molds (and slabs that divide into tall and skinny loaves) though.
Please note: This is a new PRE-SALE. ETA will be end of June or sooner. This release will only contain 50 pcs (wooden mold & blue silicone liner) This wooden soap mold has been designed with removable sides. Notched at 1" for embed decorating. Sealed with lacquer. Can be used with a silicone...winstonandwalter.ca
We also marked ours with a sharpie. But for people that get stressed over exactness and perfection like my daughter it'd make life a little easier for her and more fun. Any us suppliers would be helpful as I have tried to search and no luckTook me less than a minute to mark my mold with a Sharpie. But there are several manufactures that are doing this.