Can I suggest that as part of your App Store release process you should also release a new version to TestFlight.
I’m not suggesting you need to have any changes in the release, just a bump in the build number would do as it would stop the app from disappearing from TestFlight.
This would make life much easier when you release breaking changes such as the recent change of payment processor or for those of us who didn’t notice a standard beta update fast enough (such as when on holiday) and it should be pretty easy to setup on your continuous integration / build system.