What is purpose of `ALLOW_CONNECTIONS` being set so no one can connect to the new Postgres database? In Postgres 11, the CREATE DATABASE command that makes a new catalog has an option ALLOW_CONNECTIONS. The documentation says this option can be used to block all connections to the database:

[ ALLOW_CONNECTIONS [=] allowconn ]
allowconn
If false then no one can connect to this database. The default is true, allowing connections (except as restricted by other mechanisms, such as GRANT/REVOKE CONNECT).

➥ Why would anyone want to create a database that nobody can connect to?
➥ If you cannot connect, how can you ever change this setting to true to allow connections?
It gives you a chance to revoke connect from unwanted users before doing alter database ... allow connections

Also in all cases users with superuser permission are allowed to connect, so you can create an unconnectable database, then as superuser populate it and create constraints ans indices before making it publicly available.