Following the release of iOS 9.0.2 for iPhone, iPad, and iPod touch this morning, Apple has closed the signing window for iOS 8.4.1 and iOS 9.0. This means that users can no longer downgrade from  a more recent version of the operating system to either of those older builds.
This is something Apple always does once a new version of iOS has been out for a considerable amount of time. iOS 9 was released two weeks ago today and now that iOS 9.0.1 and iOS 9.0.2 to fix any bugs that the update may have caused, there’s no reason, at least in the eyes of Apple, for anyone to want to downgrade to iOS 8.4.1.
This may come as a disappointment to users who have been experiencing issues with iOS 9. The update caused several performance problems for users, especially those of older generation devices. Apple, however, claims that iOS 9.0.1 and iOS 9.0.2 fix nearly all of those issues.
As of right now, Apple is still signing iOS 9.0.1. So if for some reason you updated to iOS 9.0.2 this morning and need to rollback, it’s still possible at this point. It’s unclear, however, for how much longer that signing window will be open, so you should downgrade sooner rather than later if you need to.
The release of iOS 9.0.2 this morning bought a handful of bug fixes relating to iMessage, Podcasts, iCloud Drive, and more. The update also fixes the security issue that allowed access to photos and contacts from the lock screen.