Soap is much worse for your hair than sulfates. It can be much more harsh than sulfates in all scientific tests of harshness that I have seen. It is also alkaline. The pH of hair is 5.5 and most good quality cleansers are around that as well. But soap is usually 8 or more. This can damage hair badly, leaving it dull and dry. For skin soap can be fine, as skin can recover from the damage. But hair is not "alive" and cannot.
If you have hard water, soap can leave residue on your hair that's difficult to remove. Most soaps also contain heavy waxes and oils that leave even more buildup.
Some people recommend using baking soda to remove this residue which does further damage to your hair.
Initially this type of routine may make your hair look nice. But over time buildup and damage take their toll.
Curl friendly cleansers are detergents, which are different from soap.
Shampoo bars are growing in popularity. Many are touted as being environmentally friendly, natural, and budget friendly. But not all shampoo bars are made alike.
There are two main types of shampoo bars: those made with soap and those that contain detergents. Remember these are different!
I messaged Lorraine Massey about shampoo bars: she said that she believes they are just as damaging as traditional shampoo. There are a few out there made with gentler detergents (sodium cocoyl isethionate for example) that may be curl-friendly. I recommend always asking the manufacturer to confirm they are ph balanced and not made with soap since labeling can be deceptive.
Here is a post with details about soap and pH testing.
Curlsbot can detect some shampoo bars but not all based on the ingredients lists. Curlsbot only works if contains certain words in the in the ingredients list:
but not all soap ingredients lists have these ingredients. For example this list would show up OK on Curlsbot
ingredients: extra virgin olive oil, *avocado oil, castor oil, *coconut oil, pumpkinseed oil, *fair trade shea butter, purified water, sodium hydroxide, *sweet orange (citrus sinesis) essential oil, lime (citrus aurantifolia) essential oil, infused herbs: *marshmallow root, *catnip, *chamomile, * lavender
Even though it's soap! Why? The oils here have been saponified, turning them into soap.
Also occasionally Curlsbot might mark an OK ingredient as soap if it's a byproduct of the saponification process. Glycerin for example, it is a byproduct of some soapmaking, but is not soap. Please message me if you see this.
I hope to eventually train Curlsbot better to detect these patterns. Sodium hydroxide + oils is a clue but that pattern is also in many OK non-bar products, also some just list the oils especially if they are on Etsy.
I recommend you contact the manufacturer and ask if the product contains soap.