There isn't a guide for Azure AD B2C with SAP Business ByDesign specifically, but there is a general guide for how to use Azure AD B2C as the Identity Provider and secure access to SAP platforms and applications.
According to the guide, you would register a SAML application with B2C and authorize with Azure AD B2C.
Have you tried those steps? Based on the general SAP integration guide, I don't see anything that would prevent the use of Azure AD B2C as the Identity Provider, but I have reached out to the product team to see if they can provide additional guidance around this scenario and will get back with any information they provide.