The Legacy Authentication Provider type has been removed from the Authentication section of the Administrator Panel as of Learn 9.1 version Q4 2019. Users on Learn 9.1 Q4 2019 and forward can refer to the Authentication Framework for alternative options.
When upgrading from SP7 or earlier to SP8, some existing provider types are updated automatically to use the new authentication framework. These migrated providers include the default Learn Internal provider and LDAP. If your Active configured provider is not on the migrated list, it remains Active and functions as a Legacy Authentication Provider.
Using the Legacy Authentication Provider enables you to manage authentication just as it was managed in release SP7 and earlier, using the bb-config and authentication.properties files.
It is recommended when creating new authentication providers to use the Building Block authentication framework and rewrite any custom authentication modules to use the new framework. If this is not viable, you can use the legacy framework to manage authentication.