Agnostic vs Legacy Authenticators
With the release of PAS 5.1, a new authentication architecture was introduced. This is the way forward since the new authenticators are protocol agnostic and work natively with ex. SAML, OIDC, internal authentication, can be chained etc.
The old authenticators are now considered legacy authenticators - avoid these if possible for new development, and plan for a transition from them to the new architecture.