Logstash logging no longer works for the RESTBase & AQS clusters. The timing corresponds with the upgrades to Bullseye; Logstash logs stop for eaach node immediately after their upgrade. Interestingly, logging still works for the Sessionstore cluster (the first to be upgraded to Bullseye).
The logstash logback encoder hands off JSON messages to rsyslog on localhost:11514 (UDP). A tcpdump shows that Cassandra is not sending packets to rsyslog on the effected nodes.
See also:
T328490: restbase cluster: decommission end-of-life hosts
T331713: Migrate restbase servers to Bullseye
T347738: Upgrade AQS cluster to Bullseye