The mod_security is mentioned in Chapter 14. Infrastructure services | Considerations in adopting RHEL 10 | Red Hat Enterprise Linux | 10 | Red Hat Documentation
Significant changes in the package set for infrastructure services
The following packages are no longer included in Red Hat Enterprise Linux:
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Themod_securitydirective is now available in the EPEL repository .
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In other words neither RHEL 10 nor AlmaLinux 10 have that package in the base repos, but EPEL 10 should have it for them.
EPEL 10.0 does not have mod_security, but EPEL 10.1 does.
One could wait until AlmaLinux 10.1 is released (later this year),
or submit a request to EPEL to build the package also for the 10.0 targets.