Support for RHEL 8 has been announced for SCOM 2019 UR1 which should be released during Q1 2020, and may be released as well for SCOM 2016. There are very little chances SCOM 2012 gets officiel RHEL8 support since it is out of support itself, but you may
be able to unoficailly use the future 2019 agent or the Azure OMSagent when it gets support for RHEL 8 as well.