How to extract the grants on default roles of PostgreSQL 10: pg_read_all_settings, pg_read_all_stats, pg_monitor and pg_signal_backend?
Have a look at https://www.postgresql.org/docs/current/infoschema-role-table-grants.html


  The view role_table_grants identifies all privileges granted on tables
  or views where the grantor or grantee is a currently enabled role.
  Further information can be found under table_privileges. The only
  effective difference between this view and table_privileges is that
  this view omits tables that have been made accessible to the current
  user by way of a grant to PUBLIC.


See also https://stackoverflow.com/a/55459289/6368697