Apple does typically have release notes and an overview of fixed vulnerabilities for releases. I think it is likely that 11.5.2 fixes a large vulnerability and

Apple does typically have release notes and an overview of fixed vulnerabilities for releases. I think it is likely that 11.5.2 fixes a large vulnerability and they will only disclose more information once most people have updated to 11.5.2. And/or they are still preparing updates for Mojave/Catalina.