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We're in same boat as you are; very large in-house RHEL presence and I have Rancher running on RHEL VMs for couple of years now. RHEL is officially supported per their support matrix: https://www.suse.com/suse-rancher/support-matrix/all-supported-versions/rancher-v2-7-3/

The Rancher sales & support reps I've talked to have been pretty straightforward, so I suspect they'd tell you that they test and verify with RHEL and support it, but have reach to people with deeper expertise with SLES. So I don't think they'd push anyone to move, but might hint how it could go a bit better for people who are starting with no Linux (and who wants to trust IBM anyway).

Given that Rancher is a container, it shouldn't matter much and RHEL is fully supported. Not supporting RHEL would be a bold and imho stupid move.