Extended support for .NET Core 3.1 ends December 3rd 2022, so applications on Azure App Service need to be updated to .NET 6.
What does this mean?
Applications hosted on Azure App Service will still run and be functional. However, there will be no patches or customer service available. This means the applications will be open to potential security vulnerabilities or service disruptions.
What to do?
Update your applications on Azure App Service to use .NET 6 following these instructions: app-service-linux-docs/dot_net_core.md at master · Azure/app-service-linux-docs · GitHub. As well as the long-term support you will access, there are also multiple enhancements available to you on .NET 6.
Remember support for .NET Core 3.1 ends December 3rd 2022, so ideally the update should be done soon.