Chat: #wg-debian-s390x
Mailing list: https://lists.openmainframeproject.org/g/wg-debian-s390x/
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/
There is a #debian-s390x channel registered on irc.oftc.net, but it seems inactive.