The filebeat RPM seems to have gone missing from the 5.x yum repo:
# yum repository-packages logstash list all
.
.
Available Packages
elasticsearch.noarch 5.6.5-1 logstash
heartbeat.i686 5.6.5-1 logstash
heartbeat.x86_64 5.6.5-1 logstash
kibana.i686 5.6.5-1 logstash
kibana.x86_64 5.6.5-1 logstash
logstash.noarch 1:5.6.5-1 logstash
metricbeat.i686 5.6.5-1 logstash
metricbeat.x86_64 5.6.5-1 logstash
packetbeat.i686 5.6.5-1 logstash
packetbeat.x86_64 5.6.5-1 logstash