Just on the question of semver, I am not convinced that semver prohibits this.
As a user, your concerns and expectations regarding semver are about essentially how the code works, e.g. are you going to have to refactor all of your 500 dags. In other words the API.. But to me this is a lower level thing that doesn't seem should strictly be forbidden. It's like... when we change the dependencies in airflow, inevitably that will break someone's cluster if they blindly deploy to prod. But this is not a prudent thing to do. So I think it's not an unreasonable expectation that a user verify that their cluster will spin up. It's something you can verify without running any dags at all (i.e. if you set dask executor and you start airflow, i think airflow will not start). And then users can rely on semver about changing behavior, which is not something you necessarily can tell without running dags.
