ERROR: role "user1" cannot be dropped because some objects depend on it
06:42:36.347 UTC [199]: [2-1] db=cloudsqladmin,user=cloudsqlagent
DETAIL:  privileges for database mydb 24 objects in database mydb

Crossdatabase queries in PostgreSQL are not implemented. PostgreSQL will see some dependencies with the specified pg_catalog.pg_shdepend.dbid, but will not be able to query the system catalog of that database to determine which objects they are. So, postgresql does hint only database names in error details for other databases. (this is also a reason why PostgreSQL has no drop user ... cascade - we can't modify or even read objects description from another database)
But if you connect to the database specified in the details and try to delete a user, you will get a much more detailed description:
postgres=# drop user user1;
ERROR:  role &quot;user1&quot; cannot be dropped because some objects depend on it
DETAIL:  3 objects in database m2
postgres=# \connect m2 
You are now connected to database &quot;m2&quot; as user &quot;postgres&quot;.
m2=# drop user user1;
ERROR:  role &quot;user1&quot; cannot be dropped because some objects depend on it
DETAIL:  privileges for table foo
privileges for table foofk
owner of table test

In this case user1 is owner for one table (REASSIGN OWNED will solve this easily) and some priviilegies on another two tables (REASSIGN OWNED will not touch this).
Certain commands are required for different objects (revoke for permissions, alter default privileges for default privileges and so on), unfortunately, there is no omnipotent ring to manage all dependencies even in one database.