Postgres 13 - how to downgrade password requirements?
An attacker can try to enumerate which users exist by attempting to log in as a bunch of different user names and seeing what error message is reported, or how long it took to report the error message (or in some cases, how much power the server used before reporting the error or how much noise it made or things like that) for each username.  This can be useful to the attacker because he can then not waste time guessing passwords on for users which don't exist, or can spearphish for users which  he knows do exist.
To make user enumeration hard, PostgreSQL goes through a mock authentication if pg_hba says scram authentication is needed but the requested username doesn't exist; or if the user does exist but either has no password set or has a password stored in the wrong format.  The point of a mock authentication of course is that the client doesn't know it is mock.  So if the client doesn't support scram but scram is required by pg_hba, then you get this error message, regardless of how the password is actually stored.