What is the effect of setting EB "Rolling update type" to "Deactivated"?

When you set the "Rolling update type" to "Deactivated" in an Elastic Beanstalk environment, Elastic Beanstalk makes no attempt to avoid downtime during updates.

With this Deactivated setting, when you make configuration changes that modify the launch configuration or VPC settings (which require terminating and replacing all instances), Elastic Beanstalk terminates your environment's existing instances and replaces them with new instances running with the new configuration. This approach can result in downtime for your application during the update process.

The "Deactivated" setting might be useful in development or testing environments where downtime is acceptable and you want the fastest update process. However, it's generally not recommended for production environments where application availability is crucial.

Sources
Configuration changes - AWS Elastic Beanstalk