The Oracle Database 21c database initialisation parameter optimizer_real_time_statistics has been backported to Oracle Database 19c RU 10.
The real-time statistics feature is available of some platforms (as specified in the licensing manual). It is now disabled by default in Oracle Database 19c RU10 on-premises Exadata systems, but it can be enabled using the new parameter.
This means that if you are...
...then you must explicitly set optimizer_real_time_statistics = TRUE to override the new default.
The Oracle Database 19c documentation will reflect this change soon.
As per the comment below, this change is not to de-emphasize the feature: it brings it in-line with other Autonomous Database features available on Oracle Exadata. Note also that there are further enhancement to real-time statistics in Oracle Database 21c, adding number-of-distinct-value modelling.