On Oracle 18c and 19c, the CreateDBCatalog.sql script has one more line calling:
What is this pupdel.sql?
It creates a trigger after drop of user on the database, that will delete entries on SYSTEM.SQLPLUS_PRODUCT_PROFILE table belonging to this user.
But, what is SYSTEM.SQLPLUS_PRODUCT_PROFILE ?
This is the new name for PRODUCT_USER_PROFILE table, which allows to restrict usage of sqlplus based on keywords/attributes and users. PRODUCT_USER_PROFILE is now a synonym of SQLPLUS_PRODUCT_PROFILE and there is also the public view PRODUCT_PRIVS which shows the privileges of the current connected user.
From Oracle 19c all this is anyway desupported, as described at the documentation: