Apple has now stopped signing iOS 11.2.6 firmware version for all the iOS devices like iPhone, iPad and iPod Touch which means that users can no longer downgrade or restore from iOS 11.3 to iOS 11.2.6 or any lower version using iTunes.
Downgrade to iOS 11.2.6 possible?
iOS 11.2.6 is now gone and can’t be installed anyway. So whenever you try restoring your device to lower version from iTunes, you are basically forced to upgrade to iOS 11.3 directly.
Apple always gives two versions of iOS to restore whenever it releases a new version. So that people have ability to downgrade their devices to the previous version. But after some time, it stops signing the previous version.
Downgrading to iOS 11.2.6 from iOS 11.3 is not possible, but if you have SHSH for iOS 11.2.6 or below, then you can downgrade back to the same version or you have accidentally updated your device to latest version via OTA or iTunes.
You can also visit this link https://ipsw.me to check the signing status of Apple firmware.