Yeah, I find the situation here very confusing: I agree that merge keys are not part of YAML 1.2, but they are part of YAML 1.1. The reason they don't appear to be in the "main" 1.1 spec itself is because they were added to 1.1 after 1.1 was already deprecated[1].

[1]: https://ktomk.github.io/writing/yaml-anchor-alias-and-merge-...