From an email from Heroku:
PostgreSQL 9.4 reaches End of Life on 2020-Feb-13. Due to security and operational concerns, Heroku cannot run unsupported software as a service. Therefore, the following database will need to be updated before 2020-Feb-13:
You have two options:
Before 2019-Nov-13, you can schedule a time to perform an upgrade to version 9.6 or more. Typically, you will have around 10 minutes downtime. You may want additional time to ensure that everything works properly after the upgrade.
After 2019-Nov-13, we will schedule an automatic upgrade to 9.6. This forced upgrade will incur significant downtime for the database and associated app(s), which can reach 12 hours in extreme cases.
All upgrades must be completed by 2020-Feb-13. We reserve the right to deprovision any database not upgraded at that time.
We recognize that deprecations and upgrades impact your productivity and customer experience. We think it’s worth the effort to run on secure and supported software. Please email email@example.com with questions or concerns.
The Heroku Data Team
Upgrade instructions and version support: