This has been changed BACK to warning in this change https://github.com/apache/airflow/pull/21074 after this issue reported: https://github.com/apache/airflow/issues/20709
The change introduces a new type of error that a provider can raise if there is an "optional feature" that depends on not installed dependency. Previously we silenced it and it caused the #20709 kind of problems where imports for providers failed "silently" The #21704 strikes the right balance (IMHO) between "warnings on accidental import errors" and "silencing expected import errors when provider decides so". J.
