UPDATE- 12:20 PM PT on August 31st
Apple’s system status page now indicates that App Store Connect and TestFlight issues are resolved, the Apple Developer portal is also back up and running.
Apple’s developer system page also indicates that App Store, App Store Connect API, and TestFlight are experiencing problems, adding they are working to resolve it. This is likely in direct relation to the developer portal being down for maintenance as well.
The portal sometimes goes down for maintenance, however it remains unclear if this incident is related to a redesign or potential changes in the portal. A possible, but unlikely cause could be new App Store policy going live which allows developers to submit suggestions to App Store guidelines, or Apple developer tools.
This page will be updated as new information becomes available.