Per my knowledge, Azure AD B2C can not be implemented with SharePoint Online.
There are two main reasons why Azure AD B2C can't be used to authenticate users for Microsoft Office 365.
1.Microsoft Office 365 authenticates external users by using email accounts that have applied for Microsoft accounts. But with Azure AD B2C, it can use any email accounts and three-party application accounts.
2.For Microsoft Office 365, external users are managed in the same directory as internal employees. But Azure AD B2C is an independent service for building a consumer application identity repository. It makes external users store in the application directory. So, they are managed separately from the internal employees.