Update (October 21, 2025)
We have identified and fixed two additional upgrade issues. Please see our new blog posts Follow Up - Preventing Upgrade Failures from etcd v3.5 to v3.6 and Avoiding Zombie Cluster Members When Upgrading for details.
There is a common issue 19557 in the etcd v3.5 to v3.6 upgrade that may cause the upgrade process to fail. You can find detailed information and related discussions in the issue.
TL; DR
Users are required to first upgrade to etcd v3.5.20 (or a higher patch version) before upgrading to etcd v3.6.0. Failure to do so may result in an unsuccessful upgrade.