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.
tests/providers/amazon/aws/operators/test_ecs.py
self, launch_type, tags, start_mock, check_mock, wait_mock, xcom_pull_mock, xcom_del_mock
    ):
        self.set_up_operator(launch_type=launch_type, tags=tags)  
        client_mock = self.aws_hook_mock.return_value.get_conn.return_value
        client_mock.describe_task_definition.return_value = {'taskDefinition': {'family': 'f'}}
        client_mock.list_tasks.return_value = {
            'taskArns': [
                'arn:aws:ecs:us-east-1:012345678910:task/d8c67b3c-ac87-4ffe-a847-4785bc3a8b54',
                'arn:aws:ecs:us-east-1:012345678910:task/d8c67b3c-ac87-4ffe-a847-4785bc3a8b55',
            ]
        }
        self.ecs.reattach = True
        self.ecs.execute(self.mock_context)
        self.aws_hook_mock.return_value.get_conn.assert_called_once()
        extend_args = {}
        if launch_type:
            extend_args['launchType'] = launch_type
        if launch_type == 'FARGATE':
            extend_args['platformVersion'] = 'LATEST'
        if tags:
            extend_args['tags'] = [{'key': k, 'value': v} for (k, v) in tags.items()]
        client_mock.describe_task_definition.assert_called_once_with(taskDefinition='t')
        client_mock.list_tasks.assert_called_once_with(cluster='c', desiredStatus='RUNNING', family='f')
        start_mock.assert_not_called()
        xcom_pull_mock.assert_called_once_with(
            self.mock_context,
            key=self.ecs.REATTACH_XCOM_KEY,
            task_ids=self.ecs.REATTACH_XCOM_TASK_ID_TEMPLATE.format(task_id=self.ecs.task_id),
        )