Pliny the Elder wrote in the 1st century AD that soap was an "invention of the Gauls for giving a reddish tint to the hair. "
Is this mostly accepted?
Nope, it's much older than they are. More can always be said, of course, so if anyone would like to expand further on The History of Soap, please don't let this post stop you!
For the meantime, OP, here's u/Noble_Devil_Boruta explaining the history of soap, including Pliny's bit about it.