Steps to reproduce:

Start an agent in version 7.0.12 with environment variable ZBX_SERVER_HOST set

{{$ podman run -e ZBX_SERVER_HOST=0.0.0.0/0 docker.io/zabbix/zabbix-agent2:ubuntu-7.0.12                                                                                                                                                      }}
** Preparing Zabbix agent
** Preparing Zabbix agent configuration file

...
zabbix_agent2 [1]: ERROR: Cannot validate configuration: failed to validate "ServerActive" configuration parameter: failed to validate host: "0.0.0.0/0".

Do the same with 7.0.11, it works:

$ podman run -e ZBX_SERVER_HOST=0.0.0.0/0 docker.io/zabbix/zabbix-agent2:ubuntu-7.0.11

It's true that the documentation does not say tha CIDR is supported for serverActive configuration, but this behaviour change has not been reported in the release notes.

Besides, it would be useful to support this again, as some specific controlled settings (ie. kubernetes cluster deployments) could get easier if ServerActive can allow a CIDR.