"When using the SASL SCRAM mechanism, the SCRAM-SHA-256-PLUS variant SHOULD be preferred over the SCRAM-SHA-256 variant, and SHA-256 variants [RFC7677] SHOULD be preferred over SHA-1 variants [RFC5802]".

There is only SCRAM-SHA-1, there is not SCRAM-SHA-1-PLUS:

• RFC5802: Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms: https://tools.ietf.org/html/rfc5802
• RFC6120: Extensible Messaging and Presence Protocol (XMPP): Core: https://tools.ietf.org/html/rfc6120

There is not SCRAM-SHA-256(-PLUS):

• RFC7677: SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms: https://tools.ietf.org/html/rfc7677 - since 2015-11-02
• RFC8600: Using Extensible Messaging and Presence Protocol (XMPP) for Security Information Exchange: https://tools.ietf.org/html/rfc8600 - since 2019-06-21: https://mailarchive.ietf.org/arch/msg/ietf-announce/suJMmeMhuAOmGn_PJYgX5Vm8lNA

I add SCRAM-SHA-512(-PLUS): https://xmpp.org/extensions/inbox/hash-recommendations.html

-PLUS variants:

• RFC5056: On the Use of Channel Bindings to Secure Channels: https://tools.ietf.org/html/rfc5056
• RFC5929: Channel Bindings for TLS: https://tools.ietf.org/html/rfc5929
• Channel-Binding Types: https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml

LDAP:

• RFC5803: Lightweight Directory Access Protocol (LDAP) Schema for Storing Salted: Challenge Response Authentication Mechanism (SCRAM) Secrets: https://tools.ietf.org/html/rfc5803

HTTP:

• RFC7804: Salted Challenge Response HTTP Authentication Mechanism: https://tools.ietf.org/html/rfc7804

IANA:

• Simple Authentication and Security Layer (SASL) Mechanisms: https://www.iana.org/assignments/sasl-mechanisms/sasl-mechanisms.xhtml

Linked to:

• https://github.com/scram-xmpp/info/issues/1