Can I upgrade my iPad 2 to iOS 9 after the release of iOS 10?
iOS 10 is not available for the iPad 2.

Because of that, you will continue to be able to upgrade to iOS 9.3.5 at any time in the future. Apple continues to sign the last available firmware for all devices, which will be iOS 9.3.5 for the iPad 2.

Examples of how the last supported version of iOS for a device continues to be signed: here