19c is a mini-release. Remember it is a new name for the second 12cR2 patchset, after 12.2.0.2/18c
https://mikedietrichde.com/2019/02/13/oracle-database-19-2-for-exadata-is-now-available-for-download/ was the first to mention it. By looking up in the doc I found
. Distinct listagg
. Desupport sqlplus product profile
. listener.log log rotation
Go to the doc to find more https://docs.oracle.com/en/database/oracle/oracle-database/19/whats-new.html