As customer's report, the compressed type of core file created by systemd-coreump service has being changed from lz4 to zst on their RHEL9.1 system.

[root@rhela9 ~]# sleep 300 &
[1] 34939
[root@rhela9 ~]# kill -11 34939
[1]+ Segmentation fault (core dumped) sleep 300
[root@rhela9 ~]# coredumpctl
TIME PID UID GID SIG COREFILE EXE SIZE
Mon 2026-03-02 09:16:28 CST 34939 0 0 SIGSEGV present /usr/bin/sleep 17.5K
[root@rhela9 ~]# ls -l /var/lib/systemd/coredump/
total 24
rw-r----. 1 root root 17991 Mar 2 09:16 core.sleep.0.9b4c3ac37ad24afa874ca8b2a4726c7c.34939.1772414188000000.zst

But from the manual provided by all RHEL 9.0 versions and higher, the default compressed type of core files created by systemd-coredump sservice is lz4.

[root@rhela9 ~]# man core
...
Core dumps and systemd
On systems using the systemd(1) init framework, core dumps may be placed in a location determined by systemd(1). To do this, systemd(1) employs the core_pattern feature that allows piping core dumps to a program. One can verify this by checking whether core dumps are being piped to the systemd-coredump(8) program:

$ cat /proc/sys/kernel/core_pattern

In this case, core dumps will be placed in the location configured for systemd-coredump(8), typically as lz4(1) compressed files in the directory /var/lib/systemd/coredump/. One can list the core dumps that have been recorded by systemd-coredump(8) using coredumpctl(1):
...