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It is stable, as there are only stable releases listed in the Rancher Support Matrix: https://www.suse.com/suse-rancher/support-matrix/all-supported-versions/rancher-v2-9-1/ You might look into the thread above from Scott Williams where this topic is also discussed.

For whatever reason v2.9.x is not even listed on their github page: https://github.com/rancher/rancher may be 2.9.x is only for Prime customers? For example, 2.8.6 and newer and 2.7.11 and newer are not listed on github page (but listed on support matrix) because they are for Prime (paying) customers only..