1, IOS adopts the recommended upgrade strategy, forcing to keep the latest upgrade strategy. Once upgraded, it is impossible to downgrade.

2. The purpose of keeping the latest system version is to prevent the vulnerability of the lower version from leaking information to users.

3. Apple introduced the ID lock from IOS7, and it is forbidden to downgrade in order to prevent someone from stealing the mobile phone. By reusing the mobile phone through downgrade, the theft rate of the iPhone can be reduced.