Please branch and build nginx-mod-modsecurity in epel10.
yajl, a depenency required for libmodsecurity, is not available for RHEL or EPEL10 as described here: https://github.com/owasp-modsecurity/ModSecurity/issues/3308 Until libmodsecurity is ported to another yaml parser or yajl is built for EPEL10 nginx-mod-modsecurity can't be built.
I created a COPR repo: https://copr.fedorainfracloud.org/coprs/mikelo2/modsecurity-el10/