Issue
You are wondering if your app will experience downtime during the data maintenance.
Resolution
Any planned maintenance requires some downtime. If you are on a high-availability (HA) plan, you arere already aware as you have taken steps to protect yourself if an outage occurs. If you are not on a HA plan, Heroku attempts to ensure as little downtime as possible for your application. After a maintenance event, HA is unavailable on HA plans while a new HA follower is created and caught up.
During a maintenance downtime, you can see app errors such as sql_error_code = 25006 ERROR: cannot execute INSERT in a read-only transaction, or more general errors that indicate your application cannot connect to the database.