I would like to suggest for postgres_fdw: If the foreign database is PostgreSQL,
Just to be clear, the "postgres" part of the name means the remote database must be a PostgreSQL database, there is no "if". Likewise, for the extension mysql_fdw the remote database is "MySQL".
the link should just pass through all the CRUD SQL commands to the other database.
If the other database is of a version so different that cannot make sense of the CRUD SQL command, it will generate an error and that's it.
This would be very useful to keep datasets synchronized.