Currently `ECSOperator` propagates `platform_version` parameter either in case `launch_type` is `FARGATE` or there is `capacity_provider_strategy` parameter provided. The case with `capacity_provider_strategy` is wrong. Capacity provider strategy can contain a reference on EC2 capacity provider. If it's an EC2 capacity provider, then `platform_version` should not be propagated to the `boto3` api call. And it's not possible to do so with the current logic of `ECSOperator` because `platform_version` is always propagated in such case and `boto3` doesn't accept `platform_version` as `None`. So in order to fix that `platform_version` should be an independent parameter and propagated only when it's specified, regardless which `launch_type` or `capacity_provider_strategy` is specified. That should also simplify the logic of `ECSOperator`.

I will prepare a PR to fix that.
airflow/providers/amazon/aws/operators/ecs.py
:param aws_conn_id: connection id of AWS credentials / region name. If None,
        credential boto3 strategy will be used
        (http://boto3.readthedocs.io/en/latest/guide/configuration.html).
    :type aws_conn_id: str
    :param region_name: region name to use in AWS Hook.
        Override the region_name in connection (if provided)
    :type region_name: str
    :param launch_type: the launch type on which to run your task ('EC2' or 'FARGATE')
    :type launch_type: str
    :param capacity_provider_strategy: the capacity provider strategy to use for the task.
        When capacity_provider_strategy is specified, the launch_type parameter is omitted.
        If no capacity_provider_strategy or launch_type is specified,
        the default capacity provider strategy for the cluster is used.
    :type capacity_provider_strategy: list
    :param group: the name of the task group associated with the task
    :type group: str
    :param placement_constraints: an array of placement constraint objects to use for
        the task
    :type placement_constraints: list
    :param placement_strategy: an array of placement strategy objects to use for
        the task
    :type placement_strategy: list
    :param platform_version: the platform version on which your task is running
    :type platform_version: str
    :param network_configuration: the network configuration for the task
    :type network_configuration: dict
    :param tags: a dictionary of tags in the form of {'tagKey': 'tagValue'}.
    :type tags: dict
    :param awslogs_group: the CloudWatch group where your ECS container logs are stored.
        Only required if you want logs to be shown in the Airflow UI after your job has