Mailing list: https://lists.openmainframeproject.org/g/wg-linux-distros
Slack channel: #linux-distros-wg
The Debian s390x team has a mailing list at: https://lists.debian.org/debian-s390/ and their own port page at https://www.debian.org/ports/s390/
Fedora 390x is built alongside all the other architectures in the main Koji build system. It is shipped as a secondary architecture to mirrors, rather than a primary architecture.
Dan Horák is the primary contact for s390x in Fedora.