Steps to reproduce:

1. yum install https://repo.zabbix.com/zabbix/5.0/rhel/7/x86_64/zabbix-release-5.0-1.el7.noarch.rpm
2. yum search zabbix-web

Result:
1. Packages not found

2. In fact, some metapackages and packages for frontend with SCL dependencies are here, but -pgsql and -mysql packages are missing: https://repo.zabbix.com/zabbix/5.0/rhel/7/x86_64/frontend/

Solution:
1. Add missing packages
2. Update .repo in zabbix-release or repository itself to include all packages in root repodata