According to
How To Purge Listener Log.Xml File? (Doc ID 816871.1)
the log.xml file should rotate once it reaches 10 MB. But on at least two of our 19c RAC databases, I see that log.xml under <ADR Home>/alert is already way above 10 MB and yet there're no log_1.xml, log_2.xml, ...
dotrldbms5a alert $ pwd
/u01/app/oracle/diag/tnslsnr/dotrldbms5a/listener/alert
dotrldbms5a alert $ ls -l
total 95724
-rw-r----- 1 oracle oinstall 98004808 Nov 5 10:51 log.xml
We have a single-node 19c database and the log does rotate. It also does on our older (including 18c) RAC databases. In all cases, listener.ora has no extra settings other than the standard lines added by the installation programs. In sqlnet.ora, there's only one for for NAMES.DIRECTORY_PATH.