Apple removed all apps that had not been updated in a year, including SHS Assistant. There was a plan in place to keep the app running but unfortunately some new Apple Developer Guidelines made it essentially impossible.

Apple now requires Sign In With Apple to be an option on all apps using 3rd party sign in services. Since signing in with your school Google account was required to verify the student went to SHS, there was no way to add Sign In With Apple without sacrificing key features such as the school map. As such the decision was made to move onto other projects and leave SHS Assistant available to download for as long as possible.

If you still have SHS Assistant downloaded it should still work. Since the app was optimized for iOS 10 through 14, new versions may make the app increasingly unstable. However, it appears to work without significant issues on iOS 15.

UPDATE: SHS Assistant does crash significantly more on iOS 16.