GA ANNOUNCEMENT - Change ADB Mode to Read/Write or Read-Only
It is now possible to select an Autonomous Database operation mode - the default mode is Read/Write. If you select Read-Only mode users can only run queries.
In addition, for both modes it is possible to restrict access to only allow users with the RESTRICTED SESSION privilege to connect to the database. The ADMIN user has this privilege.
A typical use case for restricted access mode is to allow for administrative tasks such as indexing, data loads, or other planned activities.
For more information see here:
DOC: Change Autonomous Database Mode to Read/Write or Read-Only for ADW,