The resource owner password credentials grant MUST NOT be used. This
grant type insecurely exposes the credentials of the resource owner
to the client. Even if the client is benign, this results in an
increased attack surface (credentials can leak in more places than
just the AS) and users are trained to enter their credentials in
places other than the AS.
Furthermore, adapting the resource owner password credentials grant
to two-factor authentication, authentication with cryptographic
credentials, and authentication processes that require multiple steps
can be hard or impossible (WebCrypto, WebAuthn).
