I probably need to look more into the quirks of getting this installed on RHEL, though I think I did try RHEL 7.8 without issue.
Anyways, you can skip yum-plugin-copr by downloading the .repo file on its own… Something like:
cd /etc/yum.repos.d
curl -OL https://copr.fedorainfracloud.org/coprs/g/oisf/suricata-6.0/repo/epel-7/group_oisf-suricata-6.0-epel-7.repo